Reconstituted polypeptides

ABSTRACT

The present invention provides modified fibronectin type III (Fn3) molecules, and nucleic acid molecules encoding the modified Fn3 molecules. Also provided are methods of preparing these molecules, and kits to perform the methods.

CLAIM OF PRIORITY

This application is a continuation application of U.S. application Ser.No. 11/848,135, which was filed on Aug. 30, 2007, now U.S. Pat. No.8,258,265 which is a continuing application of U.S. application Ser. No.10/457,070, which was filed on Jun. 6, 2003, now abandoned, whichapplication claims priority to U.S. Provisional Application Ser. No.60/386,991, which was filed on Jun. 6, 2002, which applications areincorporated herein by reference.

GOVERNMENTAL RIGHTS

Portions of the present invention were made with support of the UnitedStates Government via a grant from the National Institutes of Healthunder grant numbers R29-GM 55042 and R01-DK63090, and via a grant fromthe Department of Defense under grant number DAMD17-01-1-0385. The U.S.Government therefore may have certain rights in the invention.

BACKGROUND OF THE INVENTION

U.S. patent application Ser. No. 09/096,749, which corresponds toPublication No. US 2002 0019517, and Ser. No. 09/903,412 are herebyincorporated by reference in their entirety.

Antibody Structure

A standard antibody (Ab) is a tetrameric structure consisting of twoidentical immunoglobulin (Ig) heavy chains and two identical lightchains. The heavy and light chains of an Ab consist of differentdomains. Each light chain has one variable domain (VL) and one constantdomain (CL), while each heavy chain has one variable domain (VH) andthree or four constant domains (CH) (Alzari et al., 1988). Each domain,consisting of ˜110 amino acid residues, is folded into a characteristicβ-sandwich structure formed from two β-sheets packed against each other,the immunoglobulin fold. The VH and VL domains each have threecomplementarity determining regions (CDR1-3) that are loops, or turns,connecting β-strands at one end of the domains (FIG. 1: A, C). Thevariable regions of both the light and heavy chains generally contributeto antigen specificity, although the contribution of the individualchains to specificity is not always equal. Antibody molecules haveevolved to bind to a large number of molecules by using six randomizedloops (CDRs). However, the size of the antibodies and the complexity ofsix loops represents a major design hurdle if the end result is to be arelatively small peptide ligand.

Antibody Substructures

Functional substructures of Abs can be prepared by proteolysis and byrecombinant methods. They include the Fab fragment, which contains theVH-CH1 domains of the heavy chain and the VL-CL1 domains of the lightchain joined by a single interchain disulfide bond, and the Fv fragment,which contains only the VH and VL domains. In some cases, a single VHdomain retains significant affinity (Ward et al., 1989). It has alsobeen shown that a certain monomeric κ light chain will specifically bindto its cognate antigen. (L. Masat et al., 1994). Separated light orheavy chains have sometimes been found to retain some antigen-bindingactivity (Ward et al., 1989). These antibody fragments are not suitablefor structural analysis using NMR spectroscopy due to their size, lowsolubility or low conformational stability.

Another functional substructure is a single chain Fv (scFv), made of thevariable regions of the immunoglobulin heavy and light chain, covalentlyconnected by a peptide linker (S-z Hu et al., 1996). These small (M_(r)25,000) proteins generally retain specificity and affinity for antigenin a single polypeptide and can provide a convenient building block forlarger, antigen-specific molecules. Several groups have reportedbiodistribution studies in xenografted athymic mice using scFv reactiveagainst a variety of tumor antigens, in which specific tumorlocalization has been observed. However, the short persistence of scFvsin the circulation limits the exposure of tumor cells to the scFvs,placing limits on the level of uptake. As a result, tumor uptake byscFvs in animal studies has generally been only 1-5% ID/g as opposed tointact antibodies that can localize in tumors ad 30-40% ID/g and havereached levels as high as 60-70% ID/g.

A small protein scaffold called a “minibody” was designed using a partof the Ig VH domain as the template (Pessi et al., 1993). Minibodieswith high affinity (dissociation\constant (K_(d))˜10⁻⁷ M) tointerleukin-6 were identified by randomizing loops corresponding to CDR1and CDR2 of VH and then selecting mutants using the phage display method(Martin et al., 1994). These experiments demonstrated that the essenceof the Ab function could be transferred to a smaller system. However,the minibody had inherited the limited solubility of the VH domain(Bianchi et al., 1994).

It has been reported that camels (Camelus dromedarius) often lackvariable light chain domains when IgG-like material from their serum isanalyzed, suggesting that sufficient antibody specificity and affinitycan be derived form VH domains (three CDR loops) alone. Davies andRiechmann recently demonstrated that “camelized” VH domains with highaffinity (K_(d)˜10⁻⁷ M) and high specificity can be generated byrandomizing only the CDR3. To improve the solubility and suppressnonspecific binding, three mutations were introduced to the frameworkregion (Davies & Riechmann, 1995). It has not been definitively shown,however, that camelization can be used, in general, to improve thesolubility and stability of VHs.

An alternative to the “minibody” is the “diabody.” Diabodies are smallbivalent and bispecific antibody fragments, i.e., they have twoantigen-binding sites. The fragments contain a heavy-chain variabledomain (V_(H)) connected to a light-chain variable domain (V_(L)) on thesame polypeptide chain (V_(H)-V_(L)). Diabodies are similar in size toan Fab fragment. By using a linker that is too short to allow pairingbetween the two domains on the same chain, the domains are forced topair with the complementary domains of another chain and create twoantigen-binding sites. These dimeric antibody fragments, or “diabodies,”are bivalent and bispecific (P. Holliger et al., 1993).

Since the development of the monoclonal antibody technology, a largenumber of 3D structures of Ab fragments in the complexed and/or freestates have been solved by X-ray crystallography (Webster et al., 1994;Wilson & Stanfield, 1994). Analysis of Ab structures has revealed thatfive out of the six CDRs have limited numbers of peptide backboneconformations, thereby permitting one to predict the backboneconformation of CDRs using the so-called canonical structures (Lesk &Tramontano, 1992; Rees et al., 1994). The analysis also has revealedthat the CDR3 of the VH domain (VH-CDR3) usually has the largest contactsurface and that its conformation is too diverse for canonicalstructures to be defined; VH-CDR3 is also known to have a largevariation in length (Wu et al., 1993). Therefore, the structures ofcrucial regions of the Ab-antigen interface still need to beexperimentally determined.

Comparison of crystal structures between the free and complexed stateshas revealed several types of conformational rearrangements. Theyinclude side-chain rearrangements, segmental movements, largerearrangements of VH-CDR3 and changes in the relative position of the VHand VL domains (Wilson & Stanfield, 1993). In the free state, CDRs, inparticular those which undergo large conformational changes uponbinding, are expected to be flexible. Since X-ray crystallography is notsuited for characterizing flexible parts of molecules, structuralstudies in the solution state have not been possible to provide dynamicpictures of the conformation of antigen-binding sites.

Mimicking the Antibody-Binding Site

CDR peptides and organic CDR mimetics have been made (Dougall et al.,1994). CDR peptides are short, typically cyclic, peptides whichcorrespond to the amino acid sequences of CDR loops of antibodies. CDRloops are responsible for antibody-antigen interactions. Organic CDRmimetics are peptides corresponding to CDR loops which are attached to ascaffold, e.g., a small organic compound.

CDR peptides and organic CDR mimetics have been shown to retain somebinding affinity (Smyth & von Itzstein, 1994). However, as expected,they are too small and too flexible to maintain full affinity andspecificity. Mouse CDRs have been grafted onto the human Ig frameworkwithout the loss of affinity (Jones et al., 1986; Riechmann et al.,1988), though this “humanization” does not solve the above-mentionedproblems specific to solution studies.

Mimicking Natural Selection Processes of Abs

In the immune system, specific Abs are selected and amplified from alarge library (affinity maturation). The processes can be reproduced invitro using combinatorial library technologies. The successful displayof Ab fragments on the surface of bacteriophage has made it possible togenerate and screen a vast number of CDR mutations (McCafferty et al.,1990; Barbas et al., 1991; Winter et al., 1994). An increasing number ofFabs and Fvs (and their derivatives) is produced by this technique,providing a rich source for structural studies. The combinatorialtechnique can be combined with Ab mimics.

A number of protein domains that could potentially serve as proteinscaffolds have been expressed as fusions with phage capsid proteins.Review in Clackson & Wells, Trends Biotechnol. 12:173-184 (1994).Indeed, several of these protein domains have already been used asscaffolds for displaying random peptide sequences, including bovinepancreatic trypsin inhibitor (Roberts et al., PNAS 89:2429-2433 (1992)),human growth hormone (Lowman et al., Biochemistry 30:10832-10838(1991)), Venturini et al., Protein Peptide Letters 1:70-75 (1994)), andthe IgG binding domain of Streptococcus (O'Neil et al., Techniques inProtein Chemistry V (Crabb, L,. ed.) pp. 517-524, Academic Press, SanDiego (1994)). These scaffolds have displayed a single randomized loopor region.

Researchers have used the small 74 amino acid α-amylase inhibitorTendamistat as a presentation scaffold on the filamentous phage M13(McConnell and Hoess, 1995). Tendamistat is a β-sheet protein fromStreptomyces tendae. It has a number of features that make it anattractive scaffold for peptides, including its small size, stability,and the availability of high resolution NMR and X-ray structural data.Tendamistat's overall topology is similar to that of an immunoglobulindomain, with two β-sheets connected by a series of loops. In contrast toimmunoglobulin domains, the β-sheets of Tendamistat are held togetherwith two rather than one disulfide bond, accounting for the considerablestability of the protein. By analogy with the CDR loops found inimmunoglobulins, the loops the Tendamistat may serve a similar functionand can be easily randomized by in vitro mutagenesis.

Tendamistat, however, is derived from Streptomyces tendae. Thus, whileTendamistat may be antigenic in humans, its small size may reduce orinhibit its antigenicity. Also, Tendamistat's stability is uncertain.Further, the stability that is reported for Tendamistat is attributed tothe presence of two disulfide bonds. Disulfide bonds, however, are asignificant disadvantage to such molecules in that they can be brokenunder reducing conditions and must be properly formed in order to have auseful protein structure. Further, the size of the loops in Tendamistatare relatively small, thus limiting the size of the inserts that can beaccommodated in the scaffold. Moreover, it is well known that formingcorrect disulfide bonds in newly synthesized peptides is notstraightforward. When a protein is expressed in the cytoplasmic space ofE. coli, the most common host bacterium for protein overexpression,disulfide bonds are usually not formed, potentially making it difficultto prepare large quantities of engineered molecules.

Thus, there is an on-going need for small polypeptides that bind with atarget molecule, such as an artificial antibody. These polypeptides canbe used for a variety of therapeutic, diagnostic, research and catalyticapplications. There is also an on-going need for polypeptides that bindto more than one target molecule, and for protein fragments (or bindingpairs) that associate or reconstitute to form a protein.

The following abbreviations have been used in describing amino acids,peptides, or proteins: Ala or A, Alanine; Arg or R, Arginine; Asn or Nasparagine; Asp or D, aspartic acid; Cys or C, cysteine; Gln or Q,glutamine; Glu or E, glutamic acid; Gly or G, glycine; His or H,histidine; Ile or I, isoleucine; Leu or L, leucine; Lys or K, lysine;Met or M, methionine; Phe or F, phenylalanine; Pro or P, proline; Ser orS, serine; Thr or T, threonine; Trp or W, tryptophan; Tyr or Y,tyrosine; Val or V, valine.

The following abbreviations have been used in describing nucleic acids,DNA, or RNA: A, adenosine; T, thymidine; G, guanosine; C, cytosine.

SUMMARY OF THE INVENTION

As used herein the indefinite article “a” or “an” carries the meaning of“one or more.”

Reconstitution of a protein is where two polypeptide fragments from asingle original protein are bound together, though not necessarily bycovalent bonding. “Association” is where two polypeptides fragments fromthe same or different starting proteins are bound together. Again, thebinding is not necessarily by covalent bonding. For a general discussionof protein reconstitution/reassociation, see Ojennus et al. (2001).Fragments of Fn3 will reconstitute and/or reassociate at a pH range ofbetween pH 1 and pH 10 at 30° C. At neutral pH, they will stillreassociate at 50° C.

A “coiled coil” is a widespread structural motif that is found infibrous proteins such as myosin and keratin. A coiled coil constitutestwo or more interacting α-helices, supercoiled around one another, thatare associated in a parallel or an anti-parallel orientation. Theα-helices of naturally occurring coiled coils are generally parallel.Sequence features within a natural coiled coil can lead to preferencefor an antiparallel helix orientation rather than the more commonlyobserved parallel alignment. See, Oakley and Kim, Biochemistry37:12603-12610 (1998) for a detailed discussion of coiled coils. Anotherbinding pair that could be used to encourage reassociation of twofragments is the intein system, described by Yamazaki et al., (1998).

The present invention provides an Fn3 monobody binding pair. The bindingpair is made up of two parts, a first Fn3 monobody polypeptide havingtwo to six β-strand domains (which optionally has a polypeptide tailregion attached to one or both of the terminal β-strand domains) with aloop region linked between each β-strand domain, and a second Fn3monobody polypeptide having two to six β-strand domains (whichoptionally has a polypeptide tail region attached to one or both of theterminal β-strand domains) with a loop region linked between eachβ-strand domain. A “polypeptide tail region” is a polypeptide that isone to 25 amino acids in length that is not part of the β-strand domain.A “terminal β-strand” is one of the two β-strands in the monobody thatis bound to a loop region at only one of its ends. For example, in amonobody that has three β-strands and two loop regions, one would have afirst terminal β-strand, a loop region, an internal β-strand, a loopregion, and then a second terminal β-strand. Thus, the terminalβ-strands are linked to only one loop region, whereas the internalβ-strand is linked at both ends of the β-strand.

The first Fn3 fragment associates with the second Fn3 fragment with adissociation constant of less than 10⁻⁶ moles/liter. For example, if amonobody polypeptide has two β-strand domains, it contains a single loopregion in the polypeptide; if the monobody polypeptide has threeβ-strand domains, it contains two loop regions in the polypeptide(configured such that the β-strands alternate with the loop regions); ifa monobody polypeptide has four β-strand domains, it would have threeloop regions in the polypeptide; etc. At least one loop region of thebinding pair is capable of binding to a specific binding partner (SBP)to form a polypeptide:SBP complex having a dissociation constant, asmeasured in the binding reaction of the corresponding uncut, full-lengthFN3 monobody molecule, of less than 10⁻⁶ moles/liter. The presentinvention also provides nucleic acid molecules that encode thepolypeptides that form the binding pair.

The present invention further provides methods and kits for making thepolypeptides of the binding pair. These polypeptides may contain aunique peptide sequence that can be cleaved with a specific chemicalagent such that two pre-determined peptides are generated. It is wellknown that unique peptide sequences can be cleaved with specificchemical reagents (e.g., cyanogen bromide) or with a proteases (e.g.,thrombin, enterokinase, factor X, tobacco etch virus (TEV) protease,human rhinovirus 3C protease). See, Creighton 1993, Kapust 2001,Cordingley 1990.

The first Fn3 polypeptide of the binding pair of the present inventionmay further contain a first auxiliary domain, and the second Fn3polypeptide may further contain a second auxiliary domain, wherein thefirst auxiliary domain has a binding affinity for the second auxiliarydomain with a dissociation constant of less than 10⁻⁵ moles/liter. Inone embodiment, an auxiliary region is a cysteine residue. For example,the first auxiliary region is a first cysteine and the second auxiliaryregion is a second cysteine, such that the first cysteine and the secondcysteine form a disulfide bond. Disulfide bonds would generally not bepresent in a final monobody. However, cross-linking via a disulfide bondis a good approach to enhance the assembly of complementary fragments sothat they will stay as a heterodimer. Such heterodimers are used toproduce a combinatorial library of very large size. One would performscreening of such a library using phage display or other methods. Onceone found desired monobody heterodimers (i.e., specific pairs offragments), they would be reformatted into uncut, full-length proteins.Thus, disulfide-linked monobodies are instead very useful vehicles forlibrary construction, even though disulfide linkages are not present ina final product. Cysteine residues may also be present in the loopregions and/or the β-strand regions.

In other embodiments, the auxiliary domains are a naturalprotein/peptide pair, a peptide-binding protein and its target peptide,or two fragments of a protein that have been artificially generated.Examples include coiled coils, or a C-intein and N-intein pair.

The present invention further provides a fibronectin type III (Fn3)monobody binding pair having two parts: a first fibronectin type III(Fn3) monobody polypeptide containing two to six β-strand domains with aloop region linked between each β-strand domain, wherein a polypeptidetail region is attached to one or both terminal β-strands, and a secondFn3 monobody polypeptide containing two to six β-strand domains with aloop region linked between each β-strand domain, wherein a polypeptidetail region is attached to one or both terminal β-strands, wherein thefirst Fn3 fragment associates with the second Fn3 fragment with adissociation constant of less than 10⁻⁶ moles/liter.

The present invention provides variegated nucleic acid librariesencoding Fn3 monobody polypeptides, where one or more of the loopregions of the monobody polypeptides can be modified by insertions,deletions or substitutions. The present invention also providespolypeptide libraries made from these nucleic acid libraries.

The present invention provides a fibronectin type III (Fn3) monobodypolypeptide made of two to six β-strand domains with a loop regionlinked between each β-strand domain. The monobody polypeptide is capableof binding to a target molecule with a dissociation constant of lessthan 10⁻⁶ moles/liter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. β-Strand and loop topology (A, B) and MOLSCRIPT representation(C, D; Kraiilis, 1991) of the VH domain of anti-lysozyme immunoglobulinD1.3 (A, C; Bhat et al., 1994) and 10th type III domain of humanfibronectin (B, D; Main et al., 1992). The locations of complementaritydetermining regions (CDRs, hypervariable regions) and theintegrin-binding Arg-Gly-Asp (RGD) sequence are indicated.

FIG. 2. Amino acid sequence (SEQ ID NO:110) and restriction sites of thesynthetic Fn3 gene. The residue numbering is according to Main et al.(1992). Restriction enzyme sites designed are shown above the amino acidsequence. β-Strands are denoted by underlines. The N-terminal “mq”sequence has been added for a subsequent cloning into an expressionvector (disclosed as SEQ ID NO:137). The His.tag (Novagen) fusionprotein has an additional sequence, MGSSHHHHHHSSGLVPRGSH (SEQ IDNO:114), preceding the Fn3 sequence shown above.

FIG. 3. A, Far UV CD spectra of wild-type Fn3 at 25° C. and 90° C. Fn3(50 μM) was dissolved in sodium acetate (50 mM, pH 4.6). B, thermaldenaturation of Fn3 monitored at 215 nm. Temperature was increased at arate of 1° C./min.

FIG. 4. A, Cα trace of the crystal structure of the complex of lysozyme(HEL) and the Fv fragment of the anti-hen egg-white lysozyme (anti-HEL)antibody D1.3 (Bhat et al., 1994). Side chains of the residues 99-102 ofVH CDR3, which make contact with HEL, are also shown. B, Contact surfacearea for each residue of the D1.3 VH-HEL and VH-VL interactions plottedvs. residue number of D1.3 VH. Surface area and secondary structure weredetermined using the program DSSP (Kabsh and Sander, 1983). C and D,schematic drawings of the β-sheet structure of the F strand-loop-Gstrand moieties of D1.3 VH (C) and Fn3 (D). The boxes denote residues inβ-strands and ovals those not in strands. The shaded boxes indicateresidues of which side chains are significantly buried. The broken linesindicate hydrogen bonds.

FIG. 5. Designed Fn3 gene showing DNA (SEQ ID NO:111) and amino acid(SEQ ID NO:112) sequences. The amino acid numbering is according to Mainet al. (1992). The two loops that were randomized in combinatoriallibraries are enclosed in boxes.

FIG. 6. Map of plasmid pAS45. Plasmid pAS45 is the expression vector ofHis.tag-Fn3 (6×His tag disclosed as SEQ ID NO:136).

FIG. 7. Map of plasmid pAS25. Plasmid pAS25 is the expression vector ofFn3.

FIG. 8. Map of plasmid pAS38. pAS38 is a phagmid vector for the surfacedisplay of Fn3.

FIG. 9. (Ubiquitin-1) Characterization of ligand-specific binding ofenriched clones using phage enzyme-linked immunosolvent assay (ELISA).Microliter plate wells were coated with ubiquitin (1 μg/well; “Ligand(+)) and then blocked with BSA. Phage solution in TBS containingapproximately 10¹⁰ colony forming units (cfu) was added to a well andwashed with TBS. Bound phages were detected with anti-phage antibody-PODconjugate (Pharmacia) with Turbo-TMB (Pierce) as a substrate. Absorbancewas measured using a Molecular Devices SPECTRAmax 250 microplatespectrophotometer. For a control, wells without the immobilized ligandwere used. 2-1 and 2-2 denote enriched clones from Library 2 eluted withfree ligand and acid, respectively. 4-1 and 4-2 denote enriched clonesfrom Library 4 eluted with free ligand and acid, respectively.

FIG. 10. (Ubiquitin-2) Competition phage ELISA of enriched clones. Phagesolutions containing approximately 10¹⁰ cfu were first incubated withfree ubiquitin at 4° C. for 1 hour prior to the binding to aligand-coated well. The wells were washed and phages detected asdescribed above.

FIG. 11. Competition phage ELISA of ubiquitin-binding monobody 411.Experimental conditions are the same as described above for ubiquitin.The ELISA was performed in the presence of free ubiquitin in the bindingsolution. The experiments were performed with four differentpreparations of the same clone.

FIG. 12. (Fluorescein-1) Phage ELISA of four clones, Plb25.1 (containingSEQ ID NO:115), Plb25.4 (containing SEQ ID NO:116), pLB24.1 (containingSEQ ID NO:117) and pLB24.3 (containing SEQ ID NO:118). Experimentalconditions are the same as ubiquitin-1 above.

FIG. 13. (Fluorescein-2) Competition ELISA of the four clones (SEQ IDNos:115-118). Experimental conditions are the same as ubiquitin-2 above.

FIG. 14. ¹H, ¹⁵N-HSQC spectrum of a fluorescence-binding monobodyLB25.5. Approximately 20 μM protein was dissolved in 10 mM sodiumacetate buffer (pH 5.0) containing 100 mM sodium chloride. The spectrumwas collected at 30° C. on a Varian Unity NOVA 600 NMR spectrometer.

FIG. 15. Characterization of the binding reaction of Ubi4-Fn3 to thetarget, ubiquitin. (a) Phage ELISA analysis of binding of Ubi4-Fn3 toubiquitin. The binding of Ubi4-phages to ubiquitin-coated wells wasmeasured. The control experiment was performed with wells containing noubiquitin.

(b) Competition phage ELISA of Ubi4-Fn3. Ubi4-Fn3-phages werepreincubated with soluble ubiquitin at an indicated concentration,followed by the phage ELISA detection in ubiquitin-coated wells.

(c) Competition phage ELISA testing the specificity of the Ubi4 clone.The Ubi4 phages were preincubated with 250 μg/ml of soluble proteins,followed by phage ELISA as in (b).

(d) ELISA using free proteins.

FIG. 16. Equilibrium unfolding curves for Ubi4-Fn3 (closed symbols) andwild-type Fn3 (open symbols). Squares indicate data measured in TBS(Tris HCl buffer (50 mM, pH 7.5) containing NaCl (150 mM)). Circlesindicate data measured in Gly HCl buffer (20 mM, pH 3.3) containing NaCl(300 mM). The curves show the best fit of the transition curve based onthe two-state model. Parameters characterizing the transitions arelisted in Table 8.

FIG. 17. (a) ¹H, ¹⁵N-HSQC spectrum of [¹⁵N]-Ubi4-K Fn3. (b). Difference(δ_(wild-type)-δ_(Ubi4)) of ¹H (b) and ¹⁵N (c) chemical shifts plottedversus residue number. Values for residues 82-84 (shown as filledcircles) where Ubi4-K deletions are set to zero. Open circles indicateresidues that are mutated in the Ubi4-K protein. The locations ofβ-strands are indicated with arrows.

FIG. 18. (A) Guanidine hydrochloride (GuHCl)-induced denaturation ofFNfn10 monitored by Trp fluorescence. The fluorescence emissionintensity at 355 nm is shown as a function of GuHCl concentration. Thelines show the best fits of the data to the two-state transition model.(B) Stability of FN3 at 4 M GuHCl plotted as a function of pH. (C) pHdependence of the m value.

FIG. 19. A two-dimensional H(C)CO spectrum of FNfn10 showing the ¹³Cchemical shift of the carboxyl carbon (vertical axis) and the ¹H shiftof ¹H^(β) of Asp or ¹H^(γ) of Glu, respectively (horizontal axis). Crosspeaks are labeled with their respective residue numbers.

FIG. 20. pH-Dependent shifts of the ¹³C chemical shifts of the carboxylcarbons of Asp and Glu residues in FNfn10. Panel A shows data for Asp 3,67 and 80, and Glu 38 and 47. The lines are the best fits of the data tothe Henderson-Hasselbalch equation with one ionizable group (McIntosh,L. P., Hand, G., Johnson, P. E., Joshi, M. D., Koerner, M., Plesniak, L.A., Ziser, L., Wakarchuk, W. W. & Withers, S. G. (1996) Biochemistry 35,9958-9966). Panel B shows data for Asp 7 and 23 and Glu 9. Thecontinuous lines show the best fits to the Henderson-Hasselbalchequation with two ionizable groups, while the dashed lines show the bestfits to the equation with a single ionizable group.

FIG. 21. (A) The amino acid sequence of FNfn10 (SEQ ID NO:121) shownaccording to its topology (Main, A. L., Harvey, T. S., Baron, M., Boyd,J., & Campbell, I. D. (1992) Cell 71, 671-678). Asp and Glu residues arehighlighted with gray circles. The thin lines and arrows connectingcircles indicate backbone hydrogen bonds. (B) A CPK model of FN3 showingthe locations of Asp 7 and 23 and Glu 9.

FIG. 22. Thermal denaturation of the wild-type and mutant FNfn10proteins at pH 7.0 and 2.4 in the presence of 6.3 M urea and 0.1 or 1.0M NaCl. Change in circular dichroism signal at 227 nm is plotted as afunction of temperature. The filled circles show the data in thepresence of 1 M NaCl and the open circles are data in the presence of0.1 M NaCl. The left column shows data taken at pH 2.4 and the rightcolumn at pH 7.0. The identity of proteins is indicated in the panels.

FIG. 23. GuHCl-induce denaturation of FNfn10 mutants monitored withfluorescence. Fluorescence data was converted to the fraction ofunfolded protein according to the two-state transition model (Loladze,V. V., Ibarra-Molero, B., Sanchez-Ruiz, J. M. & Makhatadze, G. I. (1999)Biochemistry 38, 16419-16423), and plotted as a function of GuHCl.

FIG. 24. pH Titration of the carboxyl ¹³C resonance of Asp and Gluresidues in D7N (open circles) and D7K (closed circles) FNfn10. Data forthe wild-type (crosses) are also shown for comparison. Residue names aredenoted in the individual panels.

FIG. 25. Topographic illustration of the sites for the introduction ofthe cleavage site insertion (GGMGG; SEQ ID NO:122) in CD and EF looprespectively (SEQ ID NO:123).

FIGS. 26A-B. Guanidine hydrochloride induced unfolding of mutantproteins with an engineered cleavage site. Circles represent proteinwithout insertion, squares an insertion in the CD loop and triangles aninsertion in the EF loop (GGMGG disclosed as SEQ ID NO:122). 26A:Comparison of insertion sites. 26B: Comparison of the effect on wildtype protein to the D7KE9Q mutant proteins. Fitting parameters for thecurves are listed in table IV-1.

FIG. 27. Cleavage of a peptide bond after methionine by cyanogenbromide.

FIG. 28. Chromatogram of the reverse phase separation of CD-loop cleavedfragments of CD92. The actual fragments are marked, additionalfractions: 1 partially uncleaved protein, 2 N-terminal fragment withHisTag leader sequence, 3 acidic degradation product of the N-terminalfragment.

FIGS. 29A-C. 29A shows gel filtration chromatograms of a mixture of N-and C-terminal at 3 μM concentration, 29B shows elution of C-terminalalone, and 29C shows elution of N-terminal alone.

FIG. 30. ¹H-¹⁵N-HSQC spectrum of uncleaved CD92 protein.

FIGS. 31A-B. ¹H-¹⁵N-HSQC spectra of the isolated C-terminal fragment at5° C. (31A) and at 30° C. (31B). At lower temperature, the fragmentappears mostly unfolded, while at higher temperatures oligomerizationrevealed additional, more dispersed peaks.

FIG. 32. ¹H-¹⁵N-HSQC spectra of the isolated N-terminal fragment at 30°C. The sample is likely in a oligomeric conformation indicated by theapparent line broadening.

FIGS. 33A-B. ¹H-¹⁵N-HSQC spectra of the N-terminal (33A) and theC-terminal fragment (33B) in partially labeled complex at 30° C.

FIGS. 34A-B. Direct comparison of the parental protein and the complexformed by the fragments. 34A shows uncut CD92 at 30° C., and 34B showsan overlay of both fragments in complex at 30° C.

FIG. 35. The ratio of 15N-NOE signal for the N-terminally labeledcomplex over that of the reference spectrum revealed that the formedcomplex is as stable as a fully folded protein. Only 5 residues show anincreased motion on the investigated timescale, most likely on eitherterminus of the fragment. The very N-terminus of FNfn10 is known to bedisordered, and the six C-terminal residues of this fragment include 4glycines (sequence GGNGGhS; SEQ ID NO:124), where (hS) stands for thehomoserine lactone that resulted in the cleavage. Error was estimatedfrom the noise in the spectra to be ±0.29.

FIG. 36. The ratio of 15N-NOE signal for the C-terminally labeledcomplex over that of the reference spectrum revealed that the formedcomplex is as stable as a fully folded protein. Error was estimated fromthe noise in the spectra to be ±0.36.

FIG. 37. Time course of the fluorescence intensity due to nonspecificadherence to the cuvette.

FIGS. 38A-F. Representative series of the reconstitution of CD92fragments monitored by fluorescence at 1M (38A, 38B), 1.5M (38C, 38D)and 2M (38E, 38F) urea. For each urea concentration, two separateexperiments are shown, each displaying fluorescence at the maximum ofthe fluorescence at 350 nm (hollow circles) and that averaged over thedata of 350 nm to 360 nm (filled diamonds), along with their respectivefitted analytical curves (lines). For the calculation, values from thefitting of the averaged curves were used.

FIG. 39. Dependence of the measured dissociation constant on ureaconcentration.

FIG. 40. Dependence of the measured dissociation constant on glycerol.

FIG. 41. Far UV CD spectra for the two fragments. Filled circlesrepresent the N-terminal, hollow ones the C-terminal.

FIG. 42: Dependence of the β-turn inflection seen in the CD spectrum ofthe C-terminal fragment. C-terminal fragment concentration are 100 μM(circles), 50 μM (squares), 10 μM (crosses) and 1.5 μM (triangles),where the lowest concentration curve was measured in buffer equal to thefluorescence experiments above. All others were measured in 20 mM sodiumphosphate buffer at pH 6. Temperature and Cooperativity of unfoldingchange with concentration.

FIG. 43. Scheme for library construction using fragment reconstitution.

FIG. 44. In vivo reconstitution of monobodies. Yeast strain EGY48 with aplasmid that encodes for the N-terminal half of the FNfn10 fused to theB42 DNA binding domain, (FNABC)-NLS-B42 fusion protein, was mated withstrain RFY206 with a plasmid that encodes for a LexA-C terminal half ofFNfn10 fusion featuring either wild-type (FNDEFG) or a monobody FG loop.FNEDFG0319 has the FG loop of monobody pYT0319, and FNEDFG4699 that ofmonobody pYT4699, which have been selected for two different targetproteins. As a control, EGY48 with pTarget plasmid (Origene) and RFY206with pBait plasmid (Origene) were used. After the mated cells werereplicated on YC Gal Raf-his-ura-trp media supplemented with 1 μM E2 andincubated over night, the b-galactosidase assay was performed usingagarose overlay method.

FIG. 45. Topographic illustration of the Fn3 molecule (SEQ ID NO:139).

FIG. 46. Schematic drawings of vectors for yeast surface display of FN3and FN3 fragments. pYDFN1 is for surface display of full-length FN3. Italso contains the X-press epitope tag, V5 epitope tag and His6 tag (SEQID NO:136) for detection of displayed FN3. pGalAgaFN(C)V5 is for surfacedisplay of an FN3 fragment (residues 43-94) that is fused to V5 and His6tags (SEQ ID NO:136). pGalsecFn(N)FLAG is for secretion of an FN3fragment (residues 1-42) that is fused to the FLAG tag.

FIG. 47. FACS analysis of surface expression of FN3 fragments. Thehorizontal axis is the fluorescence intensity of FITC, which indicatesthe amount of the FN3 C-terminal fragment that is fused to the Aga2protein and anchored on the cell surface. The vertical axis indicatesthe fluorescence intensity of PE, which indicates the amount of the FN3N-terminal fragment that is secreted as a soluble protein. Each dotrepresents one yeast cell.

(A) Yeast cells expressing the wild-type C-terminal fragment only. (B)Yeast cells expressing both the wild-type N- and C-terminal fragments.(C) Yeast cells expressing only the C-terminal fragment of thestreptavidin binding monobody, STAV1. (D) Yeast cells expressing thewild-type N-termnal fragment and the C-terminal fragment of STAV1. (E)Yeast cells expressing only the wild-type N-termnal fragment.

DETAILED DESCRIPTION OF THE INVENTION

For the past decade the immune system has been exploited as a richsource of de novo catalysts. Catalytic antibodies have been shown tohave chemoselectivity, enantioselectivity, large rate accelerations, andeven an ability to reroute chemical reactions. In most cases theantibodies have been elicited to transition state analog (TSA) haptens.These TSA haptens are stable, low-molecular weight compounds designed tomimic the structures of the energetically unstable transition statespecies that briefly (approximate half-life 10⁻¹³ s) appear alongreaction pathways between reactants and products. Anti-TSA antibodies,like natural enzymes, are thought to selectively bind and stabilizetransition state, thereby easing the passage of reactants to products.Thus, upon binding, the antibody lowers the energy of the actualtransition state and increases the rate of the reaction. These catalystscan be programmed to bind to geometrical and electrostatic features ofthe transition state so that the reaction route can be controlled byneutralizing unfavorable charges, overcoming entropic bathers, anddictating stereoelectronic features of the reaction. By this means evenreactions that are otherwise highly disfavored have been catalyzed(Janda et al. 1997). Further, in many instances catalysts have been madefor reactions for which there are no known natural or man-made enzymes.

The success of any combinatorial chemical system in obtaining aparticular function depends on the size of the library and the abilityto access its members. Most often the antibodies that are made in ananimal against a hapten that mimics the transition state of a reactionare first screened for binding to the hapten and then screened again forcatalytic activity. An improved method allows for the direct selectionfor catalysis from antibody libraries in phage, thereby linkingchemistry and replication.

A library of antibody fragments can be created on the surface offilamentous phage viruses by adding randomized antibody genes to thegene that encodes the phage's coat protein. Each phage then expressesand displays multiple copies of a single antibody fragment on itssurface. Because each phage possesses both the surface-displayedantibody fragment and the DNA that encodes that fragment, and antibodyfragment that binds to a target can be identified by amplifying theassociated DNA.

Immunochemists use as antigens materials that have as little chemicalreactivity as possible. It is almost always the case that one wishes theultimate antibody to interact with native structures. In reactiveimmunization the concept is just the opposite. One immunizes withcompounds that are highly reactive so that upon binding to the antibodymolecule during the induction process, a chemical reaction ensues. Laterthis same chemical reaction becomes part of the mechanism of thecatalytic event. In a certain sense one is immunizing with a chemicalreaction rather than a substance per se. Reactive immunogens can beconsidered as analogous to the mechanism-based inhibitors thatenzymologists use except that they are used in the inverse way in that,instead of inhibiting a mechanism, they induce a mechanism.

Man-made catalytic antibodies have considerable commercial potential inmany different applications. Catalytic antibody-based products have beenused successfully in prototype experiments in therapeutic applications,such as prodrug activation and cocaine inactivation, and innontherapeutic applications, such as biosensors and organic synthesis.

Catalytic antibodies are theoretically more attractive than noncatalyticantibodies as therapeutic agents because, being catalytic, they may beused in lower doses, and also because their effects are unusuallyirreversible (for example, peptide bond cleavage rather than binding).In therapy, purified catalytic antibodies could be directly administeredto a patient, or alternatively the patient's own catalytic antibodyresponse could be elicited by immunization with an appropriate hapten.Catalytic antibodies also could be used as clinical diagnostic tools oras regioselective or stereoselective catalysts in the synthesis of finechemicals.

I. Mutation of Fn3 Loops and Grafting of Ab Loops onto Fn3

An ideal scaffold for CDR grafting is highly soluble and stable. It issmall enough for structural analysis, yet large enough to accommodatemultiple CDRs so as to achieve tight binding and/or high specificity.

A novel strategy to generate an artificial Ab system on the framework ofan existing non-Ab protein was developed. An advantage of this approachover the minimization of an Ab scaffold is that one can avoid inheritingthe undesired properties of Abs. Fibronectin type III domain (Fn3) wasused as the scaffold. Fibronectin is a large protein which playsessential roles in the formation of extracellular matrix and cell-cellinteractions; it consists of many repeats of three types (I, II and III)of small domains (Baron et al., 1991). Fn3 itself is the paradigm of alarge subfamily (Fn3 family or s-type Ig family) of the immunoglobulinsuperfamily (IgSF). The Fn3 family includes cell adhesion molecules,cell surface hormone and cytokine receptors, chaperonins, andcarbohydrate-binding domains (for reviews, see Bork & Doolittle, 1992;Jones, 1993; Bork et al., 1994; Campbell & Spitzfaden, 1994; Harpez &Chothia, 1994).

Recently, crystallographic studies revealed that the structure of theDNA binding domains of the transcription factor NF-kB is also closelyrelated to the Fn3 fold (Ghosh et al., 1995; Müller et al., 1995). Theseproteins are all involved in specific molecular recognition, and in mostcases ligand-binding sites are formed by surface loops, suggesting thatthe Fn3 scaffold is an excellent framework for building specific bindingproteins. The 3D structure of Fn3 has been determined by NMR (Main etal., 1992) and by X-ray crystallography (Leahy et al., 1992; Dickinsonet al., 1994). The structure is best described as a β-sandwich similarto that of Ab VH domain except that Fn3 has seven β-strands instead ofnine (FIG. 1). There are three loops on each end of Fn3; the positionsof the BC, DE and FG loops approximately correspond to those of CDR1, 2and 3 of the VH domain, respectively (FIG. 1 C, D).

Fn3 is small (˜95 residues), monomeric, soluble and stable. It is one offew members of IgSF that do not have disulfide bonds; VH has aninterstrand disulfide bond (FIG. 1A) and has marginal stability underreducing conditions. Fn3 has been expressed in E. coli (Aukhil et al.,1993). In addition, 17 Fn3 domains are present just in humanfibronectin, providing important information on conserved residues whichare often important for the stability and folding (for sequencealignment, see Main et al., 1992 and Dickinson et al., 1994). Fromsequence analysis, large variations are seen in the BC and FG loops,suggesting that the loops are not crucial to stability. NMR studies haverevealed that the FG loop is highly flexible; the flexibility has beenimplicated for the specific binding of the 10th Fn3 to α₅β₁ integrinthrough the Arg-Gly-Asp (RGD) motif. In the crystal structure of humangrowth hormone-receptor complex (de Vos et al., 1992), the second Fn3domain of the receptor interacts with hormone via the FG and BC loops,suggesting it is feasible to build a binding site using the two loops.

The tenth type III module of fibronectin has a fold similar to that ofimmunoglobulin domains, with seven β strands forming two antiparallel βsheets, which pack against each other (Main et al., 1992). The structureof the type II module consists of seven β strands, which form a sandwichof two antiparallel β sheets, one containing three strands (ABE) and theother four strands (C′CFG) (Williams et al., 1988). The triple-strandedβ sheet consists of residues Glu-9-Thr-14 (A), Ser-17-Asp-23 (B), andThr-56-Ser-60 (E). The majority of the conserved residues contribute tothe hydrophobic core, with the invariant hydrophobic residues Trp-22 andTry-68 lying toward the N-terminal and C-terminal ends of the core,respectively. The β strands are much less flexible and appear to providea rigid framework upon which functional, flexible loops are built. Thetopology is similar to that of immunoglobulin C domains.

Gene Construction and Mutagenesis

A synthetic gene for tenth Fn3 of human fibronectin (FIG. 2) wasdesigned which includes convenient restriction sites for ease ofmutagenesis and uses specific codons for high-level protein expression(Gribskov et al., 1984).

The gene was assembled as follows: (1) the gene sequence was dividedinto five parts with boundaries at designed restriction sites (FIG. 2);(2) for each part, a pair of oligonucleotides that code opposite strandsand have complementary overlaps of—15 bases was synthesized; (3) the twooligonucleotides were annealed and single strand regions were filled inusing the Klenow fragment of DNA polymerase; (4) the double-strandedoligonucleotide was cloned into the pET3a vector (Novagen) usingrestriction enzyme sites at the termini of the fragment and its sequencewas confirmed by an Applied Biosystems DNA sequencer using the dideoxytermination protocol provided by the manufacturer; (5) steps 2-4 wererepeated to obtain the whole gene (plasmid pAS25) (FIG. 7).

Although the present method takes more time to assemble a gene than theone-step polymerase chain reaction (PCR) method (Sandhu et al., 1992),no mutations occurred in the gene. Mutations would likely have beenintroduced by the low fidelity replication by Taq polymerase and wouldhave required time-consuming gene editing. The gene was also cloned intothe pET15b (Novagen) vector (pEW1). Both vectors expressed the Fn3 geneunder the control of bacteriophage T7 promoter (Studier et al. 1990);pAS25 expressed the 96-residue Fn3 protein only, while pEW1 expressedFn3 as a fusion protein with poly-histidine peptide (His.tag).Recombinant DNA manipulations were performed according to MolecularCloning (Sambrook et al., 1989), unless otherwise stated.

Mutations were introduced to the Fn3 gene using either cassettemutagenesis or oligonucleotide site-directed mutagenesis techniques(Deng & Nickoloff, 1992). Cassette mutagenesis was performed using thesame protocol for gene construction described above; double-stranded DNAfragment coding a new sequence was cloned into an expression vector(pAS25 and/or pEW1). Many mutations can be made by combining a newlysynthesized strand (coding mutations) and an oligonucleotide used forthe gene synthesis. The resulting genes were sequenced to confirm thatthe designed mutations and no other mutations were introduced bymutagenesis reactions.

Design and Synthesis of Fn3 Mutants with Antibody CDRs

Two candidate loops (FG and BC) were identified for grafting. Antibodieswith known crystal structures were examined in order to identifycandidates for the sources of loops to be grafted onto Fn3. Anti-hen egglysozyme (HEL) antibody D1.3 (Bhat et al., 1994) was chosen as thesource of a CDR loop. The reasons for this choice were: (1) highresolution crystal structures of the free and complexed states areavailable (FIG. 4 A; Bhat et al., 1994), (2) thermodynamics data for thebinding reaction are available (Tello et al., 1993), (3) D1.3 has beenused as a paradigm for Ab structural analysis and Ab engineering(Verhoeyen et al., 1988; McCafferty et al., 1990) (4) site-directedmutagenesis experiments have shown that CDR3 of the heavy chain(VH-CDR3) makes a larger contribution to the affinity than the otherCDRs (Hawkins et al., 1993), and (5) a binding assay can be easilyperformed. The objective for this trial was to graft VH-CDR3 of D1.3onto the Fn3 scaffold without significant loss of stability.

An analysis of the D1.3 structure (FIG. 4) revealed that only residues99-102 (“RDYR”) (SEQ ID NO:120) make direct contact with hen egg-whitelysozyme (HEL) (FIG. 4 B), although VH-CDR3 is defined as longer (Bhatet al., 1994). It should be noted that the C-terminal half of VH-CDR3(residues 101-104) made significant contact with the VL domain (FIG. 4B). It has also become clear that D1.3 VH-CDR3 (FIG. 4 C) has a shorterturn between the strands F and G than the FG loop of Fn3 (FIG. 4 D).Therefore, mutant sequences were designed by using the RDYR (99-102)(SEQ ID NO:120) of D1.3 as the core and made different boundaries andloop lengths (Table 1). Shorter loops may mimic the D1.3 CDR3conformation better, thereby yielding higher affinity, but they may alsosignificantly reduce stability by removing wild-type interactions ofFn3.

TABLE 1 Amino acid sequences of D1.3 VH CDR3, VH8 CDR3 and Fn3 FG loop and list of planned mutants. D1.3 96   100         105(SEQ ID NO: 1) •     •           • A R E R D Y R L D Y W G Q G VH8A R G A V V S Y Y A M D Y W G Q G (SEQ ID NO: 2) Fn3     75      80         85     •         •         •Y A V T G R G D S P A S S K P I (SEQ ID NO: 3) Mutant Sequence D1.3-1Y A E R D Y R L D Y - - - - P I (SEQ ID NO: 4) D1.3-2Y A V R D Y R L D Y - - - - P I (SEQ ID NO: 5) D1.3-3Y A V R D Y R L D Y A S S K P I (SEQ ID NO: 6) D1.3-4Y A V R D Y R L D Y - - - K P I (SEQ ID NO: 7) D1.3-5Y A V R D Y R - - - - - S K P I (SEQ ID NO: 8) D 1.3-6Y A V T R D Y R L - - S S K P I (SEQ ID NO: 9) D1.3-7Y A V T E R D Y R L - S S K P I (SEQ ID NO: 10) VH8-1Y A V A V V S Y Y A M D Y - P I (SEQ ID NO: 11) VH8-2Y A V T A V V S Y Y A S S K P I (SEQ ID NO: 12) Underlines indicateresidues in β-strands. Bold characters indicate replaced residues.

In addition, an anti-HEL single VH domain termed VH8 (Ward et al., 1989)was chosen as a template. VH8 was selected by library screening and, inspite of the lack of the VL domain, VH8 has an affinity for HEL of 27nM, probably due to its longer VH-CDR3 (Table 1). Therefore, its VH-CDR3was grafted onto Fn3. Longer loops may be advantageous on the Fn3framework because they may provide higher affinity and also are close tothe loop length of wild-type Fn3. The 3D structure of VH8 was not knownand thus the VH8 CDR3 sequence was aligned with that of D1.3 VH-CDR3;two loops were designed (Table 1).

Mutant Construction and Production

Site-directed mutagenesis experiments were performed to obtain designedsequences. Two mutant Fn3s, D1.3-1 and D1.3-4 (Table 1) were obtainedand both were expressed as soluble His.tag fusion proteins. D1.3-4 waspurified and the His.tag portion was removed by thrombin cleavage.D1.3-4 is soluble up to at least 1 mM at pH 7.2. No aggregation of theprotein has been observed during sample preparation and NMR dataacquisition.

Protein Expression and Purification

E. coli BL21 (DE3) (Novagen) were transformed with an expression vector(pAS25, pEW1 and their derivatives) containing a gene for the wild-typeor a mutant. Cells were grown in M9 minimal medium and M9 mediumsupplemented with Bactotrypton (Difco) containing ampicillin (200μg/ml). For isotopic labeling, ¹⁵N NH₄Cl and/or ¹³C glucose replacedunlabeled components. 500 ml medium in a 2 liter baffle flask wereinoculated with 10 ml of overnight culture and agitated at 3TC.Isopropylthio-β-galactoside (IPTG) was added at a final concentration of1 mM to initiate protein expression when OD (600 nm) reaches one. Thecells were harvested by centrifugation 3 hours after the addition ofIPTG and kept frozen at −70° C. until used.

Fn3 without His.tag was purified as follows. Cells were suspended in 5ml/(g cell) of Tris (50 mM, pH 7.6) containingethylenediaminetetraacetic acid (EDTA; 1 mM) and phenylmethylsulfonylfluoride (1 mM). HEL was added to a final concentration of 0.5 mg/ml.After incubating the solution for 30 minutes at 3TC, it was sonicatedthree times for 30 seconds on ice. Cell debris was removed bycentrifugation. Ammonium sulfate was added to the solution andprecipitate recovered by centrifugation. The pellet was dissolved in5-10 ml sodium acetate (50 mM, pH 4.6) and insoluble material wasremoved by centrifugation. The solution was applied to a Sephacryl™S100HR column (Pharmacia) equilibrated in the sodium acetate buffer.Fractions containing Fn3 then was applied to a ResourceS® column(Pharmacia) equilibrated in sodium acetate (50 mM, pH 4.6) and elutedwith a linear gradient of sodium chloride (0-0.5 M). The protocol can beadjusted to purify mutant proteins with different surface chargeproperties.

Fn3 with His.tag was purified as follows. The soluble fraction wasprepared as described above, except that sodium phosphate buffer (50 mM,pH 7.6) containing sodium chloride (100 mM) replaced the Tris buffer.The solution was applied to a Hi-Trap™ chelating column (Pharmacia)preloaded with nickel and equilibrated in the phosphate buffer. Afterwashing the column with the buffer, His.tag-Fn3 was eluted in thephosphate buffer containing 50 mM EDTA. Fractions containing His.tag-Fn3were pooled and applied to a Sephacryl™ S100-HR column, yielding highlypure protein. The His.tag portion was cleaved off by treating the fusionprotein with thrombin using the protocol supplied by Novagen. Fn3 wasseparated from the His.tag peptide and thrombin by a ResourceS columnusing the protocol above.

The wild-type and two mutant proteins so far examined are expressed assoluble proteins. In the case that a mutant is expressed as inclusionbodies (insoluble aggregate), it is first examined if it can beexpressed as a soluble protein at lower temperature (e.g., 25-30° C.).If this is not possible, the inclusion bodies are collected by low-speedcentrifugation following cell lysis as described above. The pellet iswashed with buffer, sonicated and centrifuged. The inclusion bodies aresolubilized in phosphate buffer (50 mM, pH 7.6) containing guanidiniumchloride (GdnCl, 6 M) and will be loaded on a Hi-Trap chelating column.The protein is eluted with the buffer containing GdnCl and 50 mM EDTA.

Conformation of Mutant Fn3, D1.3-4

The ¹H NMR spectra of His.tag D1.3-4 fusion protein closely resembledthat of the wild-type, suggesting the mutant is folded in a similarconformation to that of the wild-type. The spectrum of D1.3-4 after theremoval of the His.tag peptide showed a large spectral dispersion. Alarge dispersion of amide protons (7-9.5 ppm) and a large number ofdownfield (5.0-6.5 ppm) C^(α) protons are characteristic of a β-sheetprotein (Wüthrich, 1986).

The 2D NOESY spectrum of D1.3-4 provided further evidence for apreserved conformation. The region in the spectrum showed interactionsbetween upfield methyl protons (<0.5 ppm) and methyl-methylene protons.The Val72 γ methyl resonances were well separated in the wild-typespectrum (−0.07 and 0.37 ppm; (Baron et al., 1992)). Resonancescorresponding to the two methyl protons are present in the D1.3-4spectrum (−0.07 and 0.44 ppm). The cross peak between these tworesonances and other conserved cross peaks indicate that the tworesonances in the D1.3-4 spectrum are highly likely those of Val72 andthat other methyl protons are in nearly identical environment to that ofwild-type Fn3. Minor differences between the two spectra are presumablydue to small structural perturbation due to the mutations. Val72 is onthe F strand, where it forms a part of the central hydrophobic core ofFn3 (Main et al., 1992). It is only four residues away from the mutatedresidues of the FG loop (Table 1). The results are remarkable because,despite there being 7 mutations and 3 deletions in the loop (more than10% of total residues; FIG. 12, Table 2), D1.3-4 retains a 3D structurevirtually identical to that of the wild-type (except for the mutatedloop). Therefore, the results provide strong support that the FG loop isnot significantly contributing to the folding and stability of the Fn3molecule and thus that the FG loop can be mutated extensively.

TABLE 2 Sequences of oligonucleotides Name Sequence FN1FCGGGATCCCATATGCAGGTTTCTGATGTTCCGCGTGAC CTGGAAGTTGTTGCTGCGACC(SEQ ID NO: 13) FN1R TAACTGCAGGAGCATCCCAGCTGATCAGCAGGCTAGTCGGGGTCGCAGCAACAAC (SEQ ID NO: 14) FN2FCTCCTGCAGTTACCGTGCGTTATTACCGTATCACGTAC GGTGAAACCGGTG (SEQ ID NO: 15)FN2R GTGAATTCCTGAACCGGGGAGTTACCACCGGTTTCACC G (SEQ ID NO: 16) FN3FAGGAATTCACTGTACCTGGTTCCAAGTCTACTGCTACC ATCAGCGG (SEQ ID NO: 17) FN3RGTATAGTCGACACCCGGTTTCAGGCCGCTGATGGTAGC (SEQ ID NO: 18) FN4FCGGGTGTCGACTATACCATCACTGTATACGCT (SEQ ID NO: 19) FN4RCGGGATCCGAGCTCGCTGGGCTGTCACCACGGCCAGTA ACAGCGTATACAGTGAT (SEQ ID NO: 20)FN5F CAGCGAGCTCCAAGCCAATCTCGATTAACTACCGT (SEQ ID NO: 21) FN5RCGGGATCCTCGAGTTACTAGGTACGGTAGTTAATCGA (SEQ ID NO: 22) FN5R′CGGGATCCACGCGTGCCACCGGTACGGTAGTTAATCGA (SEQ ID NO: 23) gene3FCGGGATCCACGCGTCCATTCGTTTGTGAATATCAAGGCCAATC G (SEQ ID NO: 24) gene3RCCGGAAGCTTTAAGACTCCTTATTACGCAGTATGTTAGC (SEQ ID NO: 25) 38TAABglIICTGTTACTGGCCGTGAGATCTAACCAGCGAGCTCCA (SEQ ID NO: 26) BC3GATCAGCTGGGATGCTCCTNNKNNKNNKNNKNNKTAT TACCGTATCACGTA (SEQ ID NO: 27) FG2TGTATACGCTGTTACTGGCNNKNNKNNKNNKNNKNNK NNKTCCAAGCCAATCTCGAT(SEQ ID NO: 28) FG3 CTGTATACGCTGTTACTGGCNNKNNKNNKNNKCCAGC GAGCTCCAAG(SEQ ID NO: 29) FG4 CATCACTGTATACGCTGTTACTNNKNNKNNKNNKNNKT CCAAGCCAATCTC(SEQ ID NO: 30) Restriction enzyme sites are underlined. N and K denotean equimolar mixture of A, T, G and C and that of G and T, respectively.Structure and Stability Measurements

Structures of Abs were analyzed using quantitative methods (e.g., DSSP(Kabsch & Sander, 1983) and PDBfit (D. McRee, The Scripps ResearchInstitute)) as well as computer graphics (e.g., Quanta (MolecularSimulations) and What if (G. Vriend, European Molecular BiologyLaboratory)) to superimpose the strand-loop-strand structures of Abs andFn3.

The stability of monobodies was determined by measuring temperature- andchemical denaturant-induced unfolding reactions (Pace et al., 1989). Thetemperature-induced unfolding reaction was measured using a circulardichroism (CD) polarimeter. Ellipticity at 222 and 215 nm was recordedas the sample temperature was slowly raised. Sample concentrationsbetween 10 and 50 μM were used. After the unfolding baseline wasestablished, the temperature was lowered to examine the reversibility ofthe unfolding reaction. Free energy of unfolding was determined byfitting data to the equation for the two-state transition (Becktel &Schellman, 1987; Pace et al., 1989). Nonlinear least-squares fitting wasperformed using the program Igor (WaveMetrics) on a Macintosh computer.

The structure and stability of two selected mutant Fn3s were studied;the first mutant was D1.3-4 (Table 2) and the second was a mutant calledAS40 which contains four mutations in the BC loop (A²⁶V²⁷T²⁸V²⁹)→TQRQ)(SEQ ID NO:140→141). AS40 was randomly chosen from the BC loop librarydescribed above. Both mutants were expressed as soluble proteins in E.coli and were concentrated at least to 1 mM, permitting NMR studies:

The mid-point of the thermal denaturation for both mutants wasapproximately 69° C., as compared to approximately 79° C. for thewild-type protein. The results indicated that the extensive mutations atthe two surface loops did not drastically decrease the stability of Fn3,and thus demonstrated the feasibility of introducing a large number ofmutations in both loops.

Stability was also determined by guanidinium chloride (GdnCl)- andurea-induced unfolding reactions. Preliminary unfolding curves wererecorded using a fluorometer equipped with a motor-driven syringe; GdnClor urea were added continuously to the protein solution in the cuvette.Based on the preliminary unfolding curves, separate samples containingvarying concentration of a denaturant were prepared and fluorescence(excitation at 290 nm, emission at 300-400 nm) or CD (ellipticity at 222and 215 nm) were measured after the samples were equilibrated at themeasurement temperature for at least one hour. The curve was fitted bythe least-squares method to the equation for the two-state model(Santoro & Bolen, 1988; Koide et al., 1993). The change in proteinconcentration was compensated if required.

Once the reversibility of the thermal unfolding reaction is established,the unfolding reaction is measured by a Microcal MC-2 differentialscanning calorimeter (DSC). The cell (˜1.3 ml) will be filled with FnAbsolution (0.1-1 mM) and ΔCp (=ΔH/ΔT) will be recorded as the temperatureis slowly raised. T_(m) (the midpoint of unfolding), ΔH of unfolding andΔG of unfolding is determined by fitting the transition curve (Privalov& Potekhin, 1986) with the Origin software provided by Microcal.

Thermal Unfolding

A temperature-induced unfolding experiment on Fn3 was performed usingcircular dichroism (CD) spectroscopy to monitor changes in secondarystructure. The CD spectrum of the native Fn3 shows a weak signal near222 nm (FIG. 3A), consistent with the predominantly β-structure of Fn3(Perczel et al., 1992). A cooperative unfolding transition is observedat 80-90° C., clearly indicating high stability of Fn3 (FIG. 3B). Thefree energy of unfolding could not be determined due to the lack of apost-transition baseline. The result is consistent with the highstability of the first Fn3 domain of human fibronectin (Litvinovich etal., 1992), thus indicating that Fn3 domains are in general highlystable.

Binding Assays

The binding reactions of monobodies were characterized quantitativelyusing an isothermal titration calorimeter (ITC) and fluorescencespectroscopy.

The enthalpy change (AH) of binding were measured using a Microcal OmegaITC (Wiseman et al., 1989). The sample cell (˜1.3 ml) was filled withMonobody solution (≦100 μM, changed according to K_(d)), and thereference cell filled with distilled water; the system was equilibratedat a given temperature until a stable baseline is obtained; 5-20 μl ofligand solution (≦2 mM) was injected by a motor-driven syringe within ashort duration (20 sec) followed by an equilibration delay (4 minutes);the injection was repeated and heat generation/absorption for eachinjection was measured. From the change in the observed heat change as afunction of ligand concentration, ΔH and K_(d) was determined (Wisemanet al., 1989). ΔG and ΔS of the binding reaction was deduced from thetwo directly measured parameters. Deviation from the theoretical curvewas examined to assess nonspecific (multiple-site) binding. Experimentswere also be performed by placing a ligand in the cell and titratingwith an FnAb. It should be emphasized that only ITC gives directmeasurement of ΔH, thereby making it possible to evaluate enthalpic andentropic contributions to the binding energy. ITC was successfully usedto monitor the binding reaction of the D1.3 Ab (Tello et al., 1993; Bhatet al., 1994).

Intrinsic fluorescence is monitored to measure binding reactions withK_(d) in the sub-μM range where the determination of K_(d) by ITC isdifficult. Trp fluorescence (excitation at ˜290 nm, emission at 300-350nm) and Tyr fluorescence (excitation at ˜260 nm, emission at ˜303 nm) ismonitored as the Fn3-mutant solution (≦10 μM) is titrated with ligandsolution (≦100 μM). K_(d) of the reaction is determined by the nonlinearleast-squares fitting of the bimolecular binding equation. Presence ofsecondary binding sites is examined using Scatchard analysis. In allbinding assays, control experiments are performed busing wild-type Fn3(or unrelated monobodies) in place of monobodies of interest.

II. Production of Fn3 Mutants with High Affinity and SpecificityMonobodies

Library screening was carried out in order to select monobodies thatbind to specific ligands. This is complementary to the modeling approachdescribed above. The advantage of combinatorial screening is that onecan easily produce and screen a large number of variants (≧10⁸), whichis not feasible with specific mutagenesis (“rational design”)approaches. The phage display technique (Smith, 1985; O'Neil & Hoess,1995) was used to effect the screening processes. Fn3 was fused to aphage coat protein (pIII) and displayed on the surface of filamentousphages. These phages harbor a single-stranded DNA genome that containsthe gene coding the Fn3 fusion protein. The amino acid sequence ofdefined regions of Fn3 were randomized using a degenerate nucleotidesequence, thereby constructing a library. Phages displaying Fn3 mutantswith desired binding capabilities were selected in vitro, recovered andamplified. The amino acid sequence of a selected clone can be identifiedreadily by sequencing the Fn3 gene of the selected phage. The protocolsof Smith (Smith & Scott, 1993) were followed with minor modifications.

The objective was to produce Monobodies which have high affinity tosmall protein ligands. BEL and the B1 domain of staphylococcal protein G(hereafter referred to as protein G) were used as ligands. Protein G issmall (56 amino acids) and highly stable (Minor & Kim, 1994; Smith etal., 1994). Its structure was determined by NMR spectroscopy (Gronenbornet al., 1991) to be a helix packed against a four-strand β-sheet. Theresulting FnAb-protein G complexes (˜150 residues) is one of thesmallest protein-protein complexes produced to date, well within therange of direct NMR methods. The small size, the high stability andsolubility of both components and the ability to label each with stableisotopes (¹³C and ¹⁵N; see below for protein G) make the complexes anideal model system for NMR studies on protein-protein interactions.

The successful loop replacement of Fn3 (the mutant D1.3-4) demonstratethat at least ten residues can be mutated without the loss of the globalfold. Based on this, a library was first constructed in which onlyresidues in the FG loop are randomized. After results of loopreplacement experiments on the BC loop were obtained, mutation siteswere extended that include the BC loop and other sites.

Construction of Fn3 Phage Display System

An M13 phage-based expression vector pASM1 has been constructed asfollows: an oligonucleotide coding the signal peptide of OmpT was clonedat the 5′ end of the Fn3 gene; a gene fragment coding the C-terminaldomain of M13 pIII was prepared from the wild-type gene III gene of M13mp18 using PCR (Corey et al., 1993) and the fragment was inserted at the3′ end of the OmpT-Fn3 gene; a spacer sequence has been inserted betweenFn3 and pIII. The resultant fragment (OmpT-Fn3-pIII) was cloned in themultiple cloning site of M13 mp18, where the fusion gene is under thecontrol of the lac promoter. This system will produce the Fn3-pIIIfusion protein as well as the wild-type pIII protein. The co-expressionof wild-type pIII is expected to reduce the number of fusion pIIIprotein, thereby increasing the phage infectivity (Corey et al., 1993)(five copies of pIII are present on a phage particle). In addition, asmaller number of fusion pIII protein may be advantageous in selectingtight binding proteins, because the chelating effect due to multiplebinding sites should be smaller than that with all five copies of fusionpIII (Bass et al., 1990). This system has successfully displayed theserine protease trypsin (Corey et al., 1993). Phages were produced andpurified using E. coli K91kan (Smith & Scott, 1993) according to astandard method (Sambrook et al., 1989) except that phage particles werepurified by a second polyethylene glycol precipitation and acidprecipitation.

Successful display of Fn3 on fusion phages has been confirmed by ELISAusing an Ab against fibronectin (Sigma), clearly indicating that it isfeasible to construct libraries using this system.

An alternative system using the fUSE5 (Parmley & Smith, 1988) may alsobe used. The Fn3 gene is inserted to fUSE5 using the SfiI restrictionsites introduced at the 5′- and 3′-ends of the Fn3 gene PCR. This systemdisplays only the fusion pIII protein (up to five copies) on the surfaceof a phage. Phages are produced and purified as described (Smith &Scott, 1993). This system has been used to display many proteins and isrobust. The advantage of fUSE5 is its low toxicity. This is due to thelow copy number of the replication form (RF) in the host, which in turnmakes it difficult to prepare a sufficient amount of RF for libraryconstruction (Smith & Scott, 1993).

Construction of Libraries

The first library was constructed of the Fn3 domain displayed on thesurface of M13 phage in which seven residues (77-83) in the FG loop(FIG. 4D) were randomized. Randomization will be achieved by the use ofan oligonucleotide containing degenerated nucleotide sequence. Adouble-stranded nucleotide was prepared by the same protocol as for genesynthesis (see above) except that one strand had an (NNK)₆(NNG) sequenceat the mutation sites, where N corresponds to an equimolar mixture of A,T, G and C and K corresponds to an equimolar mixture of G and T. The(NNG) codon at residue 83 was required to conserve the SacI restrictionsite (FIG. 2). The (NNK) codon codes all of the 20 amino acids, whilethe NNG codon codes 14. Therefore, this library contained ˜10⁹independent sequences. The library was constructed by ligating thedouble-stranded nucleotide into the wild-type phage vector, pASM1, andthe transfecting E. coli XL1 blue (Stratagene) using electroporation.XL1 blue has the lacI^(q) phenotype and thus suppresses the expressionof the Fn3-pIII fusion protein in the absence of lac inducers. Theinitial library was propagated in this way, to avoid selection againsttoxic Fn3-pIII clones. Phages displaying the randomized Fn3-pIII fusionprotein were prepared by propagating phages with K91kan as the host.K91kan does not suppress the production of the fusion protein, becauseit does not have lacI^(q). Another library was also generated in whichthe BC loop (residues 26-20) was randomized.

Selection of Displayed Monobodies

Screening of Fn3 phage libraries was performed using the biopanningprotocol (Smith & Scott, 1993); a ligand is biotinylated and the strongbiotin-streptavidin interaction was used to immobilize the ligand on astreptavidin-coated dish. Experiments were performed at room temperature(˜22° C.). For the initial recovery of phages from a library, 10 μg of abiotinylated ligand were immobilized on a streptavidin-coatedpolystyrene dish (35 mm, Falcon 1008) and then a phage solution(containing ˜10¹¹ pfu (plaque-forming unit)) was added. After washingthe dish with an appropriate buffer (typically TBST, Tris-HCl (50 mM, pH7.5), NaCl (150 mM) and Tween 20 (0.5%)), bound phages were eluted byone or combinations of the following conditions: low pH, an addition ofa free ligand, urea (up to 6 M) and, in the case of anti-protein GMonobodies, cleaving the protein G-biotin linker by thrombin. Recoveredphages were amplified using the standard protocol using K91kan as thehost (Sambrook et al., 1989). The selection processes were repeated 3-5times to concentrate positive clones. From the second round on, theamount of the ligand were gradually decreased (to ˜1 μg) and thebiotinylated ligand were mixed with a phage solution before transferringa dish (G. P. Smith, personal communication). After the final round,10-20 clones were picked, and their DNA sequence will be determined. Theligand affinity of the clones were measured first by the phage-ELISAmethod (see below).

To suppress potential binding of the Fn3 framework (background binding)to a ligand, wild-type Fn3 may be added as a competitor in the buffers.In addition, unrelated proteins (e.g., bovine serum albumin, cytochromec and RNase A) may be used as competitors to select highly specificMonobodies.

Binding Assay

The binding affinity of Monobodies on phage surface is characterizedsemi-quantitatively using the phage ELISA technique (Li et al., 1995).Wells of microtiter plates (Nunc) are coated with a ligand protein (orwith streptavidin followed by the binding of a biotinylated ligand) andblocked with the Blotto solution (Pierce). Purified phages (˜10¹⁰ pfu)originating from single plaques (M13)/colonies (fUSE5) are added to eachwell and incubated overnight at 4° C. After washing wells with anappropriate buffer (see above), bound phages are detected by thestandard ELISA protocol using anti-M13 Ab (rabbit, Sigma) andanti-rabbit Ig-peroxidase conjugate (Pierce) or using anti-M13Ab-peroxidase conjugate (Pharmacia). Colormetric assays are performedusing TMB (3,3′,5,5′-tetramethylbenzidine, Pierce). The high affinity ofprotein G to immunoglobulins presents a special problem; Abs cannot beused in detection. Therefore, to detect anti-protein G Monobodies,fusion phages are immobilized in wells and the binding is then measuredusing biotinylated protein G followed by the detection usingstreptavidin-peroxidase conjugate.

Production of Soluble Monobodies

After preliminary characterization of mutant Fn3s using phage ELISA,mutant genes are subcloned into the expression vector pEW1. Mutantproteins are produced as His.tag fusion proteins and purified, and theirconformation, stability and ligand affinity are characterized.

III. Increased Stability of Fn3 Scaffolds

The definition of “higher stability” of a protein is the ability of aprotein to retain its three-dimensional structure required for functionat a higher temperature (in the case of thermal denaturation), and inthe presence of a higher concentration of a denaturing chemical reagentsuch as guanidine hydrochloride. This type of “stability” is generallycalled “conformational stability.” It has been shown that conformationalstability is correlated with resistance against proteolytic degradation,i.e., breakdown of protein in the body (Kamtekar et al. 1993).

Improving the conformational stability is a major goal in proteinengineering. Here, mutations have been developed by the inventor thatenhance the stability of the fibronectin type III domain (Fn3). Theinventor has developed a technology in which Fn3 is used as a scaffoldto engineer artificial binding proteins (Koide et al., 1998). It hasbeen shown that many residues in the surface loop regions of Fn3 can bemutated without disrupting the overall structure of the Fn3 molecule,and that variants of Fn3 with a novel binding function can be engineeredusing combinatorial library screening (Koide et al., 1998). The inventorfound that, wild-type Fn3 is an excellent scaffold, Fn3 variants thatcontain large number of mutations are destabilized against chemicaldenaturation, compared to the wild-type Fn3 protein (Koide et al.,1998). Thus, as the number of mutated positions are mutated in order toengineer a new binding function, the stability of such Fn3 variantsfurther decreases, ultimately leading to marginally stable proteins.Because artificial binding proteins must maintain theirthree-dimensional structure to be functional, stability limits thenumber of mutations that can be introduced in the scaffold. Thus,modifications of the Fn3 scaffold that increase its stability are usefulin that they allow one to introduce more mutations for better function,and that they make it possible to use Fn3-based engineered proteins in awider range of applications.

The inventor found that wild-type Fn3 is more stable at acidic pH thanat neutral pH (Koide et al., 1998). The pH dependence of Fn3 stabilityis characterized in FIG. 18. The pH dependence curve has an apparenttransition midpoint near pH 4 (FIG. 18). These results suggest that byidentifying and removing destabilizing interactions in Fn3 one is ableto improve the stability of Fn3 at neutral pH. It should be noted thatmost applications of engineered Fn3, such as diagnostics, therapeuticsand catalysts, are expected to be used near neutral pH, and thus it isimportant to improve the stability at neutral pH. Studies by otherinvestigators have demonstrated that the optimization of surfaceelectrostatic properties can lead to a substantial increase in proteinstability (Perl et al. 2000, Spector et al. 1999, Loladze et al. 1999,Grimsley et al. 1999).

The pH dependence of Fn3 stability suggests that amino acids with pK_(a)near 4 are involved in the observed transition. The carboxyl groups ofaspartic acid (Asp) and glutamic acid (Glu) have pK_(a) in this range(Creighton, T. E. 1993). It is well known that if a carboxyl group hasunfavorable (i.e. destabilizing) interactions in a protein, its pK_(a)is shifted to a higher value from its standard, unperturbed value (Yangand Honig 1992). Thus, the pK_(a) values of all carboxyl groups in Fn3were determined using nuclear magnetic resonance (NMR) spectroscopy, toidentify carboxyl groups with unusual pK_(a)'s, as shown below.

First, the ¹³C resonance for the carboxyl carbon of each Asp and Gluresidue were assigned (FIG. 19). Next pH titration of ¹³C resonances wasperformed for these groups (FIG. 20). The pK_(a) values for theseresidues are listed in Table 3.

TABLE 3 pK_(a) values for Asp and Glu residues in Fn3. Residue pK_(a) E95.09 E38 3.79 E47 3.94 D3 3.66 D7 3.54, 5.54* D23 3.54, 5.25* D67 4.18D80 3.40 The standard deviation in the pK_(a) values are less than 0.05pH units. *Data for D7 and D23 were fitted with a transition curve withtwo pK_(a) values.

These results show that Asp 7 and 23, and Glu 9 have up-shifted pK_(a)'swith respect to their unperturbed pK_(a)'s (approximately 4.0),indicating that these residues are involved in unfavorable interactions.In contrast, the other Asp and Glu residues have pK_(a)'s close to therespective unperturbed values, indicating that the carboxyl groups ofthese residues do not significantly contribute to the stability of Fn3.

In the three-dimensional structure of Fn3 (Main et al. 1992), Asp 7 and23, and Glu 9 form a patch on the surface (FIG. 21), with Asp 7centrally located in the patch. This spatial proximity of thesenegatively charged residues explains why these residues have unfavorableinteractions in Fn3. At low pH where these residues are protonated andneutral, the unfavorable interactions are expected to be mostlyrelieved. At the same time, the structure suggests that the stability ofFn3 at neutral pH could be improved if the electrostatic repulsionbetween these three residues is removed. Because Asp 7 is centrallylocated among the three residues, it was decided to mutate Asp 7. Twomutants were prepared, D7N and D7K (i.e., the aspartic acid at aminoacid residue number 7 was substituted with an asparagine residue or alysine residue, respectively). The former replaces the negative chargewith a neutral residue of virtually the same size. The latter places apositive charge at residue 7.

The degrees of stability of the mutant proteins were characterized inthermal and chemical denaturation measurements. In thermal denaturationmeasurements, denaturation of the Fn3 proteins was monitored usingcircular dichroism spectroscopy at the wavelength of 227 nm. All theproteins underwent a cooperative transition (FIG. 22). From thetransition curves, the midpoints of the transition (T_(m)) for thewild-type, D7N and D7K were determined to be 62, 69 and 70° C. in 0.02 Msodium phosphate buffer (pH 7.0) containing 0.1 M sodium chloride and6.2 M urea. Thus, the mutations increased the T_(m) of wild-type Fn3 by7-8° C.

Chemical denaturation of Fn3 proteins was monitored using fluorescenceemission from the single Trp residue of Fn3 (FIG. 23). The free energiesof unfolding in the absence of guanidine)HCl (ΔG⁰) were determined to be7.4, 8.1 and 8.0 kcal/mol for the wild-type, D7N and D7K, respectively(a larger ΔG⁰) indicates a higher stability). The two mutants were againfound to be more stable than the wild-type protein.

These results show that a point mutation on the surface cansignificantly enhance the stability of Fn3. Because these mutations areon the surface, they minimally alter the structure of Fn3, and they canbe easily introduced to other, engineered Fn3 proteins. In addition,mutations at Glu 9 and/or Asp 23 also enhance the stability of Fn3.Furthermore, mutations at one or more of these three residues can becombined.

Thus, Fn3 is the fourth example of a monomeric immunoglobulin-likescaffold that can be used for engineering binding proteins. Successfulselection of novel binding proteins have also been based on minibody,tendamistat and “camelized” immunoglobulin VH domain scaffolds (Martinet al., 1994; Davies & Riechmann, 1995; McConnell & Hoess, 1995). TheFn3 scaffold has advantages over these systems. Bianchi et al. reportedthat the stability of a minibody was 2.5 kcal/mol, significantly lowerthan that of Ubi4-K. No detailed structural characterization ofminibodies has been reported to date. Tendamistat and the VH domaincontain disulfide bonds, and thus preparation of correctly foldedproteins may be difficult. Davies and Riechmann reported that the yieldsof their camelized VH domains were less than 1 mg per liter culture(Davies & Riechmann, 1996).

Thus, the Fn3 framework can be used as a scaffold for molecularrecognition. Its small size, stability and well-characterized structuremake Fn3 an attractive system. In light of the ubiquitous presence ofFn3 in a wide variety of natural proteins involved in ligand binding,one can engineer Fn3-based binding proteins to different classes oftargets.

IV. Reassociation of the Fibronectin Type III Domain by FragmentComplementation

Specific binding molecules are useful for many purposes. One example ofspecific binding molecules is antibodies generated by the immune system.When an individual is exposed to a “foreign” target molecule, theindividual's immune system usually produces antibodies specific for thetarget molecule. Antibodies, or other specific binding molecules, can beuseful in laboratory and commercial settings as well. At times,particular antibodies can be isolated from animals that have beenexposed to certain target molecules. It can also be useful to generateartificially assembled libraries of specific binding molecules, whichare then screened for their abilities to bind to different targetmolecules.

Phage display selection (Rader and Barbas 1997; Hoess 2001) and yeasttwo-hybrid assays (Fields and Song 1989; Geyer and Brent 2000) are amongthe most widely used experiments for the selection of proteins from alibrary. Protein selection mirrors the process in the immune responsethat selects circulating antibodies having an affinity for a particularantigen. The transformation efficiency of the host organism used to makethe library, however, limits the available size of the library. In orderto expand the binding capabilities and/or efficiencies, mutations can beintroduced into a protein sequence after an initial selection (Hawkinset al. 1992; Roberts et al. 1996; Patten et al. 1996). This methodmirrors somatic mutations in the immune response in affinity maturationexperiments.

One source of diversity in the immune response lies in the combinationof the light and heavy chains to form an antibody. Proper assembly of alight and heavy chain pair is required for the protein to be functional.Successful assembly of the heavy and light chains produced from a singlevector has been demonstrated (Barbas et al. 1991), and phage displaymethods have been developed that make it possible to “mix and match” theheavy and light chains to produce a diverse set of antibodies (Sblatteroand Bradbury 2000; Sblattero et at 2001). In contrast toimmunoglobulins, most engineered binding proteins, including monobodies,are based on a monomeric proteins (Skerra 2000). This monomeric natureof these engineered binding proteins makes it difficult to exploreheterodimerization reactions to increase the diversity of a library.However, if an engineered binding protein could be manipulated so thatits two separate pieces self-assemble into a functional form, a morediverse library could be achieved By using such a two-part scheme, twofragments could be separately diversified to generate their respectivelibraries, and then the two libraries could be combined to produce avery large library of the reconstituted protein.

Protein Fragment Reconstitution

When a protein is cleaved in two fragments, individual fragments areusually unfolded, and they often fail to reconstitute the original foldwhen mixed. However, fragments of a number of proteins have been shownto reconstitute into a native-like complex (de Prat Gay and Fersht 1994;Kippen et al. 1994; Tasayco and Chao 1995; Ladurner et at 1997;Pelletier et at 1998; Tasayco et al. 2000; Berggard et al. 2001). Inorder to achieve fragment complementation with a low dissociationconstant (i.e., high affinity), it is imperative that a protein iscleaved at a location that does not disrupt interactions important forthe stability of the protein. Most cleavage reactions in successfulreconstitution experiments have been placed in a flexible region oftarget proteins.

Use of Fragment Reconstitution in Protein Interaction Assays

Several screening strategies exploit fragment reconstitution ofproteins. For example, the split ubiquitin assay (Johnsson andVarshaysky 1994; Raquet et al. 2001) allows in vivo detection ofprotein-protein interactions (Wittke et al. 1999). A bacterial fragmentcomplementation assay (Pelletier et al. 1998; Michnick et al. 2000)similarly uses fragment reconstitution of dihydrofolate reductase toexamine protein-protein interactions in E. coli. In these assays, twoproteins of interest are respectively fused to complementary fragmentsof a reporter protein (ubiquitin or dihydrofolate reductase) andsuccessful reconstitution of the reporter protein would indicateinteraction between the two proteins.

Protein Reconstitution to Efficiently Generate Combinatorial Libraries

Many molecular display technologies, where genetic information andfunctional information are physically linked, such as phage display (Kayet al. 1996) and yeast display (Boder and Wittrup 1997) depend ontransformation of microbes. Such transformation step tends to limit thenumber of independent clones in a library that can be generated in asingle transformation reaction to ˜409 in Escherichia coli and ˜107 inyeast. If one wishes to generate a biological combinatorial librarywhere two discrete segments of a protein are diversified, one wouldtypically need to generate a single DNA vector in which two segments arediversified and transform bacteria (or yeast). In this case the librarysize is still limited by the efficiency of the transformation reaction.One could use in vivo recombination reactions to increase the librarydiversity as demonstrated for antibody fragments (Sblattero and Bradbury2000; Sblattero et al. 2001). However, when applied to a monomericprotein, such approaches introduce artificial amino acid segment in theprotein, whose effects on the stability and structure are unpredictable.

The production of diverse combinatorial libraries would be greatlysimplified, and expanded, if one could reconstitute a binding proteinfrom two physically separate libraries, as in the combination of lightand heavy chains described above. A library of a protein reconstitutedfrom two libraries of complementary fragments has an effective size ofthe product of the sizes of the two primary libraries. Thus, if one canefficiently combine (and reconstitute) two (or more) fragment libraries,the resulting library would have much greater diversity than the sum ofthe diversity of the fragment libraries. In this scheme, the fragmentsmust reconstitute with high affinity. Mutations introduced into eitherfragment potentially decrease the affinity of reconstitution. If a highaffinity reconstitution library could be engineered using a particularscaffold, intriguing new opportunities to dramatically increase thelibrary diversity would open up. Because fragments of a protein do notoften reconstitute with high affinity and specificity, experimentalstudies are needed to explore this possibility for specific proteinsystems of interest.

As discussed above, the tenth fibronectin type III domain of humanfibronectin (FNfn10) is a small, monomeric β-sandwich protein, similarto immunoglobulins. Small antibody mimics have been made using FNfn10 asa scaffold. As discussed herein, mutations are introduced into variousloop regions of the fold. Fragments of FNfn10 that were produced bycleavage of a peptide bond in the CD loop and EF loop were tested todetermine whether reconstitution occurs.

As described above, monobodies are engineered binding proteins using thescaffold of the fibronectin type III domain (FN3). Surface loopsconnecting beta-strands were modified to confer novel binding function.The present inventor has further developed the monobody technology andshowed that monobodies that bind to a given target can be engineered byscreening combinatorial libraries in which amino acid residues in one ormore surface loops are diversified. Monobodies are compatible withvirtually any molecular display techniques including, but not limitedto, phage display, yeast surface display, mRNA display and also yeasttwo-hybrid techniques.

As described above, the efficiency of introducing a nucleic acid library(transformation) in a host usually limits the achievable size of abiological library. For example, one can only construct a phage displaylibrary (host: E. coli) of ˜10⁹ independent clones and a yeasttwo-hybrid library (host: yeast) of −10⁷ independent clones from asingle transformation reaction. Theoretically libraries containing 10⁹and −10⁷ clones include all possible sequences for only 6 and 4randomized positions, respectively. Because the present inventortypically diversified more than six positions, typical monobodylibraries contained only a small fraction of possible sequences. Thismay lead to a failure to identify a monobody that binds to a target, ora failure to isolate the optimal monobody. Thus, it is of considerableinterest to increase the size of a biological library.

In the present invention, a method has been developed to significantlyincrease the size of a monobody library. This method exploits thereconstitution of a monobody from two fragments, where each contains oneor more functional loops for target binding (see FIG. 43). Acombinatorial library is made for each fragment, and then a final,larger library is constructed by combining libraries for the fragments.This is conceptually analogous to the formation of immunoglobulins fromtwo separate chains, the heavy and light chains. This strategy isparticularly suited for the yeast surface display and yeast two-hybridmethod, because yeast cells of opposite mating types can mateefficiently. For example, if one has a library for the N-terminal halfof the monobody with a size of 10⁵ and a library for the C-terminal halfwith a size of 10⁵, combining these two will theoretically yield alibrary of 10¹⁰. This is at least 10,000 fold greater than the typicalsize of a single library constructed in yeast. One can apply the in vivorecombination techniques (Sblattero and Bradbury 2000; Sblattero et at2001) to a plasmid vector containing two separate genes, where each geneencodes a fragment of a monobody. This is possible because one caninsert arbitrary DNA sequences between the two genes for the fragmentswithout causing deleterious effects on the protein.

In order to achieve this reconstitution method, it first needed to bedemonstrated that two fragments of FN3 can actually reconstitute. Sincefragment reconstitution with high affinity does not always occur,experimental verification was necessary. First, one needs to decidewhere to cut the FN3 scaffold. The CD loop was chosen as the initial cutsite. The CD loop is at the opposite end of the protein from the BC andFG loops that have been extensively used for binding. Using a yeasttwo-hybrid system the inventor confirmed that the wild-type N-terminalfragment (“FNABC”) interacted with the wild-type and mutated C-terminalfragments (“FNDEFG” and its derivatives) (FIG. 1). In addition, datasuggested that the dissociation constant (Kd) between FNABC and FN DEFGwas in the single nanomolar range, indicating very tight and specificinteraction (see EXAMPLE XXI). The present results demonstrated thatwhen cut in the CD loop, the two fragments of FN3 can reconstitute withhigh affinity. Thus monobody libraries can be constructed using thefragment reconstitution strategy.

In certain situations, particular mutations in the monobody (e.g. in theBC, DE or FG loops) may have detrimental effects on reconstitution. Insuch a situation, it is possible to attach a heterodimerization motif(such as coiled coil; see e.g. McClain et al., 2001) at the C-terminusof FNABC and at the N-terminus of FNDEFG to augment the reconstitutionaffinity of the two fragments. Alternatively, an N-intein can beattached to the end of one of the fragment pair, and a C-intein attachedto the end of the other half of the fragment pair to reconstitute thebinding protein into one contiguous polypeptide (see e.g., Yamazaki etal. 1998). Furthermore, a cystein residue can be introduced in eachfragment in such a way that a disulfide bond is formed between the twocomplementary fragments.

There are many other different classes of binding pairs that couldpotentially be used to augment the reconstitution affinity of monobodyfragments. Examples include the following:

-   1. natural proteins/peptides that are known to associate    -   coiled coils (Oakley & Kim)    -   nuclease-nuclease inhibitor (e.g., Barnase and Barster, see the        World-Wide-Web at    -   ncbi.nlm.nih        gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=7789535&dopt=Abstract)-   2. a peptide-binding protein and its target peptides    -   src homology 3 (SH3) domain and proline-rich peptides (Yu et        al., see the World-Wide-Web at    -   ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=7510218&dopt=Abstract)    -   WW domain and proline-rich peptides (Chen et al.)    -   src homology 2 (SH2) domain and phosphotyrosine containing        peptides (see the World-Wide-Web at        ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9383403&dopt=Abstract)-   3. fragments of a protein that have been artificially generated    (similar to the Fn3 fragments discussed extensively in the present    specification)    -   chymotrypsin inhibitor 2 (Ladurner et al. 1997)    -   barnase (Sancho, J. & Fersht, A. R. see the World-Wide-Web at        ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=1569553&dopt=Abstract)    -   ribonuclease S(S-protein/S-peptide) (Dwyer et al. 2001)    -   green fluorescence protein (Merkel and Regan 2000)-   4. inteins (Yamazaki et al. 1998)    Fragments of FN3 as a Heterodimerization Unit

Complementary fragments of FN3 (“split FN3”) can also be exploited asheterodimerization motifs that bring two proteins of interest in closeproximity. Two proteins of interest, X and Y, are each fused to afragment of FN3. Upon association of the FN3 fragments, X and Y are heldin close proximity. For this purpose, FN3 fragments are derived from thewild-type sequence of FN3, as demonstrated in Example XXII, or fromvariants of FN3 with increased stability. In addition, mutations areintroduced such that the new mutant fragments associate, but they do notassociate with fragments derived from the wild-type sequence (seeExample XXIII). Multiple sets of such unique binding pairs are designedusing this strategy. Such pairs can be generated by first introducing ahighly destabilizing mutations in one fragment and then screen a libraryof the other fragment in which appropriate positions are diversified.One can use this system to examine effects of bringing two proteinstogether in cell biology (Fujiwara et al. 2002). One can use this systemto assemble nanostructures, such as on a silicon surface. In thenanotechnology field, there are not many tools to attach pieces withhigh selectivity. Having many different building blocks is clearlyuseful when assembling complex structures that require differentattachment tools.

The following examples are intended to illustrate but not limit theinvention.

Example I Construction of the Fn3 Gene

A synthetic gene for tenth Fn3 of fibronectin (FIG. 1) was designed onthe basis of amino acid residue 1416-1509 of human fibronectin(Komblihtt, et al., 1985) and its three dimensional structure (Main, etal., 1992). The gene was engineered to include convenient restrictionsites for mutagenesis and the so-called “preferred codons” for highlevel protein expression (Gribskov, et al., 1984) were used. Inaddition, a glutamine residue was inserted after the N-terminalmethionine in order to avoid partial processing of the N-terminalmethionine which often degrades NMR spectra (Smith, et al., 1994).Chemical reagents were of the analytical grade or better and purchasedfrom Sigma Chemical Company and J. T. Baker, unless otherwise noted.Recombinant DNA procedures were performed as described in “MolecularCloning” (Sambrook, et al., 1989), unless otherwise stated. Customoligonucleotides were purchased from Operon Technologies. Restrictionand modification enzymes were from New England Biolabs.

The gene was assembled in the following manner First, the gene sequence(FIG. 5) was divided into five parts with boundaries at designedrestriction sites: fragment 1, NdeI-PstI (oligonucleotides FN1F and FN1R(Table 2); fragment 2, PstI-EcoRI (FN2F and FN2R); fragment 3,EcoRI-SalI (FN3F and FN3R); fragment 4, SalI-SacI (FN4F and FN4R);fragment 5, SacI-BamHI (FN5F and FN5R). Second, for each part, a pair ofoligonucleotides which code opposite strands and have complementaryoverlaps of approximately 15 bases was synthesized. Theseoligonucleotides were designated FN1F-FN5R and are shown in Table 2.Third, each pair (e.g., FN1F and FN1R) was annealed and single-strandregions were filled in using the Klenow fragment of DNA polymerase.Fourth, the double stranded oligonucleotide was digested with therelevant restriction enzymes at the termini of the fragment and clonedinto the pBlueScript® SK plasmid (Stratagene®) which had been digestedwith the same enzymes as those used for the fragments. The DNA sequenceof the inserted fragment was confirmed by DNA sequencing using anApplied Bio systems DNA sequencer and the dideoxy termination protocolprovided by the manufacturer. Last, steps 2-4 were repeated to obtainthe entire gene.

The gene was also cloned into the pET3a and pET15b (Novagen) vectors(pAS45 and pAS25, respectively). The maps of the plasmids are shown inFIGS. 6 and 7. E. coli BL21 (DE3) (Novagen) containing these vectorsexpressed the Fn3 gene under the control of bacteriophage T7 promotor(Studier, et al., 1990); pAS24 expresses the 96-residue Fn3 proteinonly, while pAS45 expresses Fn3 as a fusion protein with poly-histidinepeptide (His.tag). High level expression of the Fn3 protein and itsderivatives in E. coli was detected as an intense band on SDS-PAGEstained with CBB.

The binding reaction of the monobodies is characterized quantitativelyby means of fluorescence spectroscopy using purified soluble monobodies.

Intrinsic fluorescence is monitored to measure binding reactions. Trpfluorescence (excitation at ˜290 nm, emission at 300 350 nm) and Tyrfluorescence (excitation at ˜260 nm, emission at ˜303 nm) is monitoredas the Fn3-mutant solution (≦100 μM) is titrated with a ligand solution.When a ligand is fluorescent (e.g. fluorescein), fluorescence from theligand may be used. K_(d) of the reaction will be determined by thenonlinear least-squares fitting of the bimolecular binding equation.

If intrinsic fluorescence cannot be used to monitor the bindingreaction, monobodies are labeled with fluorescein-NHS (Pierce) andfluorescence polarization is used to monitor the binding reaction (Burkeet al., 1996).

Example II Modifications to Include Restriction Sites in the Fn3 Gene

The restriction sites were incorporated in the synthetic Fn3 genewithout changing the amino acid sequence Fn3. The positions of therestriction sites were chosen so that the gene construction could becompleted without synthesizing long (>60 bases) oligonucleotides and sothat two loop regions could be mutated (including by randomization) bythe cassette mutagenesis method (i.e., swapping a fragment with anothersynthetic fragment containing mutations). In addition, the restrictionsites were chosen so that most sites were unique in the vector for phagedisplay. Unique restriction sites allow one to recombine monobody cloneswhich have been already selected in order to supply a larger sequencespace.

Example III Construction of M13 Phage Display Libraries

A vector for phage display, pAS38 (for its map, see FIG. 8) wasconstructed as follows. The XbaI-BamHI fragment of pET12a encoding thesignal peptide of OmpT was cloned at the 5′ end of the Fn3 gene. TheC-terminal region (from the FN5F and FN5R oligonucleotides, see Table 2)of the Fn3 gene was replaced with a new fragment consisting of the FN5Fand FN5R′ oligonucleotides (Table 2) which introduced a MluI site and alinker sequence for making a fusion protein with the pIII protein ofbacteriophage M13. A gene fragment coding the C-terminal domain of M13pIII was prepared from the wild-type gene III of M13 mp18 using PCR(Corey, et al., 1993) and the fragment was inserted at the 3′ end of theOmpT-Fn3 fusion gene using the MluI and HindIII sites.

Phages were produced and purified using a helper phage, M13K07,according to a standard method (Sambrook, et al., 1989) except thatphage particles were purified by a second polyethylene glycolprecipitation. Successful display of Fn3 on fusion phages was confirmedby ELISA (Harlow & Lane, 1988) using an antibody against fibronectin(Sigma) and a custom anti-FN3 antibody (Cocalico Biologicals, PA, USA).

Example IV Libraries Containing Loop Variegations in the AB Loop

A nucleic acid phage display library having variegation in the AB loopis prepared by the following methods. Randomization is achieved by theuse of oligonucleotides containing degenerated nucleotide sequence.Residues to be variegated are identified by examining the X-ray and NMRstructures of Fn3 (Protein Data Bank accession numbers, 1FNA and 1TTF,respectively). Oligonucleotides containing NNK (N and K here denote anequimolar mixture of A, T, G, and C and an equimolar mixture of G and T,respectively) for the variegated residues are synthesized (seeoligonucleotides BC3, FG2, FG3, and FG4 in Table 2 for example). The NNKmixture codes for all twenty amino acids and one termination codon(TAG). TAG, however, is suppressed in the E. coli XL-1 blue.Single-stranded DNAs of pAS38 (and its derivatives) are prepared using astandard protocol (Sambrook, et al., 1989).

Site-directed mutagenesis is performed following published methods (seefor example, Kunkel, 1985) using a Muta-Gene® kit (BioRad). Thelibraries are constructed by electroporation of E. coli XL-1 Blueelectroporation competent cells (200 μl, Stratagene) with 1 μg of theplasmid DNA using a BTX electrocell manipulator ECM 395 1 mm gapcuvette. A portion of the transformed cells is plated on an LB-agarplate containing ampicillin (100 μg/ml) to determine the transformationefficiency. Typically, 3×10⁸ transformants are obtained with 1 μg ofDNA, and thus a library contains 10⁸ to 10⁹ independent clones. Phagemidparticles were prepared as described above.

Example V

Loop Variegations in the BC, CD, DE, EF or FG Loop

A nucleic acid phage display library having five variegated residues(residues number 26-30) in the BC loop, and one having seven variegatedresidues (residue numbers 78-84) in the FG loop, was prepared using themethods described in Example IV above. Other nucleic acid phage displaylibraries having variegation in the CD, DE or EF loop can be prepared bysimilar methods.

Example VI Loop Variegations in the FG and BC Loop

A nucleic acid phage display library having seven variegated residues(residues number 78-84) in the FG loop and five variegated residues(residue number 26-30) in the BC loop was prepared. Variegations in theBC loop were prepared by site-directed mutagenesis (Kunkel, et al.)using the BC3 oligonucleotide described in Table 1. Variegations in theFG loop were introduced using site-directed mutagenesis using the BCloop library as the starting material, thereby resulting in librariescontaining variegations in both BC and FG loops. The oligonucleotide FG2has variegating residues 78-84 and oligonucleotide FG4 has variegatingresidues 77-81 and a deletion of residues 82-84.

A nucleic acid phage display library having five variegated residues(residues 78-84) in the FG loop and a three residue deletion (residues82-84) in the FG loop, and five variegated residues (residues 26-30) inthe BC loop, was prepared. The shorter FG loop was made in an attempt toreduce the flexibility of the FG loop; the loop was shown to be highlyflexible in Fn3 by the NMR studies of Main, et al. (1992). A highlyflexible loop may be disadvantageous to forming a binding site with ahigh affinity (a large entropy loss is expected upon the ligand binding,because the flexible loop should become more rigid). In addition, otherFn3 domains (besides human) have shorter FG loops (for sequencealignment, see FIG. 12 in Dickinson, et al. (1994)).

Randomization was achieved by the use of oligonucleotides containingdegenerate nucleotide sequence (oligonucleotide BC3 for variegating theBC loop and oligonucleotides FG2 and FG4 for variegating the FG loops).

Site-directed mutagenesis was performed following published methods (seefor example, Kunkel, 1985). The libraries were constructed byelectrotransforming E. coli XL-1 Blue (Stratagene). Typically a librarycontains 10⁸ to 10⁹ independent clones. Library 2 contains fivevariegated residues in the BC loop and seven variegated residues in theFG loop. Library 4 contains five variegated residues in each of the BCand FG loops, and the length of the FG loop was shortened by threeresidues.

Example VII fd Phage Display Libraries Constructed with LoopVariegations

Phage display libraries are constructed using the fd phage as thegenetic vector. The Fn3 gene is inserted in fUSE5 (Parmley & Smith,1988) using SfiI restriction sites which are introduced at the 5′ and 3′ends of the Fn3 gene using PCR. The expression of this phage results inthe display of the fusion pIII protein on the surface of the fd phage.Variegations in the Fn3 loops are introduced using site-directedmutagenesis as described hereinabove, or by subcloning the Fn3 librariesconstructed in M13 phage into the fUSE5 vector.

Example VIII Other Phage Display Libraries

T7 phage libraries (Novagen, Madison, Wis.) and bacterial piliexpression systems (Invitrogen) are also useful to express the Fn3 gene.

Example IX Isolation of Polypeptides which Bind to MacromolecularStructures

The selection of phage-displayed monobodies was performed following theprotocols of Barbas and coworkers (Rosenblum & Barbas, 1995). Briefly,approximately 1 μg of a target molecule (“antigen”) in sodium carbonatebuffer (100 mM, pH 8.5) was immobilized in the wells of a microtiterplate (Maxisorp, Nunc) by incubating overnight at 4° C. in an air tightcontainer. After the removal of this solution, the wells were thenblocked with a 3% solution of BSA (Sigma, Fraction V) in TBS byincubating the plate at 37° C. for 1 hour. A phagemid library solution(50 μl) containing approximately 10¹² colony forming units (cfu) ofphagemid was absorbed in each well at 37° C. for 1 hour. The wells werethen washed with an appropriate buffer (typically TBST, 50 mM Tris-HCl(pH 7.5), 150 mM NaCl, and 0.5% Tween20) three times (once for the firstround). Bound phage were eluted by an acidic solution (typically, 0.1 Mglycine-HCl, pH 2.2; 50 μl) and recovered phage were immediatelyneutralized with 3 μl of Tris solution. Alternatively, bound phage wereeluted by incubating the wells with 50 μl of TBS containing the antigen(1-10 μM). Recovered phage were amplified using the standard protocolemploying the XL1Blue cells as the host (Sambrook, et al.). Theselection process was repeated 5-6 times to concentrate positive clones.After the final round, individual clones were picked and their bindingaffinities and DNA sequences were determined.

The binding affinities of monobodies on the phage surface werecharacterized using the phage ELISA technique (Li, et al., 1995). Wellsof microtiter plates (Nunc) were coated with an antigen and blocked withBSA. Purified phages (10⁸-10¹¹ cfu) originating from a single colonywere added to each well and incubated 2 hours at 37° C. After washingwells with an appropriate buffer (see above), bound phage were detectedby the standard ELISA protocol using anti-M13 antibody (rabbit, Sigma)and anti-rabbit Ig-peroxidase conjugate (Pierce). Colorimetric assayswere performed using Turbo-TMB (3,3′,5,5′-tetramethylbenzidine, Pierce)as a substrate.

The binding affinities of monobodies on the phage surface were furthercharacterized using the competition ELISA method (Djavadi-Ohaniance, etal., 1996). In this experiment, phage ELISA is performed in the samemanner as described above, except that the phage solution contains aligand at varied concentrations. The phage solution was incubated a 4°C. for one hour prior to the binding of an immobilized ligand in amicrotiter plate well. The affinities of phage displayed monobodies areestimated by the decrease in ELISA signal as the free ligandconcentration is increased.

After preliminary characterization of monobodies displayed on thesurface of phage using phage ELISA, genes for positive clones weresubcloned into the expression vector pAS45. E. coli BL21(DE3) (Novagen)was transformed with an expression vector (pAS45 and its derivatives).Cells were grown in M9 minimal medium and M9 medium supplemented withBactotryptone (Difco) containing ampicillin (200 μg/ml). For isotopiclabeling, ¹⁵N NH₄Cl and/or ¹³C glucose replaced unlabeled components.Stable isotopes were purchased from Isotec and Cambridge Isotope Labs.500 ml medium in a 21 baffle flask was inoculated with 10 ml ofovernight culture and agitated at approximately 140 rpm at 37° C. IPTGwas added at a final concentration of 1 mM to induce protein expressionwhen OD(600 nm) reached approximately 1.0. The cells were harvested bycentrifugation 3 hours after the addition of IPTG and kept frozen at−70° C. until used.

Fn3 and monobodies with His.tag were purified as follows. Cells weresuspended in 5 ml/(g cell) of 50 mM Tris (pH 7.6) containing 1 mMphenylmethylsulfonyl fluoride. HEL (Sigma, 3× crystallized) was added toa final concentration of 0.5 mg/ml. After incubating the solution for 30min at 37° C., it was sonicated so as to cause cell breakage three timesfor 30 seconds on ice. Cell debris was removed by centrifugation at15,000 rpm in an Sorval RC-2B centrifuge using an SS-34 rotor.Concentrated sodium chloride is added to the solution to a finalconcentration of 0.5 M. The solution was then applied to a 1 ml HisTrap™chelating column (Pharmacia) preloaded with nickel chloride (0.1 M, 1ml) and equilibrated in the Tris buffer (50 mM, pH 8.0) containing 0.5 Msodium chloride. After washing the column with the buffer, the boundprotein was eluted with a Tris buffer (50 mM, pH 8.0) containing 0.5 Mimidazole. The His.tag portion was cleaved off, when required, bytreating the fusion protein with thrombin using the protocol supplied byNovagen (Madison., Wis.). Fn3 was separated from the His.tag peptide andthrombin by a Resources® column (Pharmacia) using a linear gradient ofsodium chloride (0-0.5 M) in sodium acetate buffer (20 mM, pH 5.0).

Small amounts of soluble monobodies were prepared as follows. XL-1 Bluecells containing pAS38 derivatives (plasmids coding Fn3-pIII fusionproteins) were grown in LB media at 37° C. with vigorous shaking untilOD(600 nm) reached approximately 1.0; IPTG was added to the culture to afinal concentration of 1 mM, and the cells were further grown overnightat 37° C. Cells were removed from the medium by centrifugation, and thesupernatant was applied to a microtiter well coated with a ligand.Although XL-1 Blue cells containing pAS38 and its derivatives expressFN3-pIII fusion proteins, soluble proteins are also produced due to thecleavage of the linker between the Fn3 and pIII regions by proteolyticactivities of E. coli (Rosenblum & Barbas, 1995). Binding of a monobodyto the ligand was examined by the standard ELISA protocol using a customantibody against Fn3 (purchased from Cocalico Biologicals, Reamstown,Pa.). Soluble monobodies obtained from the periplasmic fraction of E.coli cells using a standard osmotic shock method were also used.

Example X Ubiquitin Binding Monobody

Ubiquitin is a small (76 residue) protein involved in the degradationpathway in eurkaryotes. It is a single domain globular protein. Yeastubiquitin was purchased from Sigma Chemical Company and was used withoutfurther purification.

Libraries 2 and 4, described in Example VI above, were used to selectubiquitin-binding monobodies. Ubiquitin (1 μg in 50 μl sodiumbicarbonate buffer (100 mM, pH 8.5)) was immobilized in the wells of amicrotiter plate, followed by blocking with BSA (3% in TBS). Panning wasperformed as described above. In the first two rounds, 1 μg of ubiquitinwas immobilized per well, and bound phage were elute with an acidicsolution. From the third to the sixth rounds, 0.1 μg of ubiquitin wasimmobilized per well and the phage were eluted either with an acidicsolution or with TBS containing 10 μM ubiquitin.

Binding of selected clones was tested first in the polyclonal mode,i.e., before isolating individual clones. Selected clones from alllibraries showed significant binding to ubiquitin. These results areshown in FIG. 9. The binding to the immobilized ubiquitin of the cloneswas inhibited almost completely by less than 30 μM soluble ubiquitin inthe competition ELISA experiments (see FIG. 10). The sequences of the BCand FG loops of ubiquitin-binding monobodies is shown in Table 4.

TABLE 4 Sequences of ubiquitin-binding monobodies Occurrence (if moreName BC loop FG loop than one) 211 CWRRA RWIPLAK 2 (SEQ ID NO: 31)(SEQ ID  NO: 32) 212 CWRRA RWVGLAW (SEQ ID NO: 33) (SEQ ID  NO: 34) 213CKHRR FADLWWR (SEQ ID NO: 35) (SEQ ID  NO: 36) 214 CRRGR RGFMWLS(SEQ ID NO: 37) (SEQ ID  NO: 38) 215 CNWRR RAYRYRW (SEQ ID NO: 39)(SEQ ID  NO: 40) 411 SRLRR PPWRV 9 (SEQ ID NO: 41) (SEQ ID  NO: 42) 422ARWTL RRWWW (SEQ ID NO: 43) (SEQ ID  NO: 44) 424 GQRTF RRWWA(SEQ ID NO: 45) (SEQ ID  NO: 46) The 411 clone, which was the mostenriched clone, was characterized using phage ELISA. The 411 cloneshowed selective binding and inhibition of binding in the presence ofabout 10 /M ubiquitin in solution (FIG. 11).

Example XI Methods for the Immobilization of Small Molecules

Target molecules were immobilized in wells of a microtiter plate(Maxisorp, Nunc) as described hereinbelow, and the wells were blockedwith BSA. In addition to the use of carrier protein as described below,a conjugate of a target molecule in biotin can be made. The biotinylatedligand can then be immobilized to a microtiter plate well which has beencoated with streptavidin.

In addition to the use of a carrier protein as described below, onecould make a conjugate of a target molecule and biotin (Pierce) andimmobilize a biotinylated ligand to a microtiter plate well which hasbeen coated with streptavidin (Smith and Scott, 1993).

Small molecules may be conjugated with a carrier protein such as bovineserum albumin (BSA, Sigma), and passively adsorbed to the microtiterplate well. Alternatively, methods of chemical conjugation can also beused. In addition, solid supports other than microtiter plates canreadily be employed.

Example XII Fluorescein Binding Monobody

Fluorescein has been used as a target for the selection of antibodiesfrom combinatorial libraries (Barbas, et al. 1992). NHS-fluorescein wasobtained from Pierce and used according to the manufacturer'sinstructions in preparing conjugates with BSA (Sigma). Two types offluorescein-BSA conjugates were prepared with approximate molar ratiosof 17 (fluorescein) to one (BSA).

The selection process was repeated 5-6 times to concentrate positiveclones. In this experiment, the phage library was incubated with aprotein mixture (BSA, cytochrome C (Sigma, Horse) and RNaseA (Sigma,Bovine), 1 mg/ml each) at room temperature for 30 minutes, prior to theaddition to ligand coated wells. Bound phage were eluted in TBScontaining 10 μM soluble fluorescein, instead of acid elution. After thefinal round, individual clones were picked and their binding affinities(see below) and DNA sequences were determined.

TABLE 5 BC FG Clones from Library #2 WT AVTVR (SEQ ID NO: 47) RGDSPAS(SEQ ID NO: 48) pLB24.1 CNWRR (SEQ ID NO: 49) RAYRYRW (SEQ ID NO: 50)pLB24.2 CMWRA (SEQ ID NO: 51) RWGMLRR (SEQ ID NO: 52) pLB24.3 ARMRE(SEQ ID NO: 53) RWLRGRY (SEQ ID NO: 54) pLB24.4 CARRR (SEQ ID NO: 55)RRAGWGW (SEQ ID NO: 56) pLB24.5 CNWRR (SEQ ID NO: 57) RAYRYRW(SEQ ID NO: 58) pLB24.6 RWRER (SEQ ID NO: 59) RHPWTER (SEQ ID NO: 60)pLB24.7 CNWRR (SEQ ID NO: 61) RAYRYRW (SEQ ID NO: 62) pLB24.8 ERRVP(SEQ ID NO: 63) RLLLWQR (SEQ ID NO: 64) pLB24.9 GRGAG (SEQ ID NO: 65)FGSFERR (SEQ ID NO: 66) pLB24.11 CRWTR (SEQ ID NO: 67) RRWFDGA(SEQ ID NO: 68) pLB24.12 CNWRR (SEQ ID NO: 69) RAYRYRW (SEQ ID NO: 70)Clones from Library #4 WT AVTVR (SEQ ID NO: 71) GRGDS (SEQ ID NO: 72)pLB25.1 GQRTF (SEQ ID NO: 73) RRWWA (SEQ ID NO: 74) pLB25.2 GQRTF(SEQ ID NO: 75) RRWWA (SEQ ID NO: 76) pLB25.3 GQRTF (SEQ ID NO: 77)RRWWA (SEQ ID NO: 78) pLB25.4 LRYRS (SEQ ID NO: 79) GWRWR(SEQ ID NO: 80) pLB25.5 GQRTF (SEQ ID NO: 81) RRWWA (SEQ ID NO: 82)pLB25.6 GQRTF (SEQ ID NO: 83) RRWWA (SEQ ID NO: 84) pLB25.7 LRYRS(SEQ ID NO: 85) GWRWR (SEQ ID NO: 86) pLB25.9 LRYRS (SEQ ID NO: 87)GWRWR (SEQ ID NO: 88) pLB25.11 GQRTF (SEQ ID NO: 89) RRWWA(SEQ ID NO: 90) pLB25.12 LRYRS (SEQ ID NO: 91) GWRWR (SEQ ID NO: 92)

Preliminary characterization of the binding affinities of selectedclones were performed using phage ELISA and competition phage ELISA (seeFIG. 12 (Fluorescein-1) and FIG. 13 (Fluorescein-2)). The four clonestested showed specific binding to the ligand-coated wells, and thebinding reactions are inhibited by soluble fluorescein (see FIG. 13).

Example XIII Digoxigenin Binding Monobody

Digoxigenin-3-O-methyl-carbonyl-e-aminocapronic acid-NHS (BoehringerMannheim) is used to prepare a digoxigenin-BSA conjugate. The couplingreaction is performed following the manufacturers' instructions. Thedigoxigenin-BSA conjugate is immobilized in the wells of a microliterplate and used for panning. Panning is repeated 5 to 6 times to enrichbinding clones. Because digoxigenin is sparingly soluble in aqueoussolution, bound phages are eluted from the well using acidic solution.See Example XIV.

Example XIV TSAC (Transition State Analog Compound) Binding Monobodies

Carbonate hydrolyzing monobodies are selected as follows. A transitionstate analog for carbonate hydrolysis, 4-nitrophenyl phosphonate issynthesized by an Arbuzov reaction as described previously (Jacobs andSchultz, 1987). The phosphonate is then coupled to the carrier protein,BSA, using carbodiimide, followed by exhaustive dialysis (Jacobs andSchultz, 1987). The hapten-BSA conjugate is immobilized in the wells ofa microliter plate and monobody selection is performed as describedabove. Catalytic activities of selected monobodies are tested using4-nitrophenyl carbonate as the substrate.

Other haptens useful to produce catalytic monobodies are summarized inH. Suzuki (1994) and in N. R. Thomas (1994).

Example XV NMR Characterization of Fn3 and Comparison of the Fn3Secreted by Yeast with that Secreted by E. Coli

Nuclear magnetic resonance (NMR) experiments are performed to identifythe contact surface between FnAb and a target molecule, e.g., monobodiesto fluorescein, ubiquitin, RNaseA and soluble derivatives ofdigoxigenin. The information is then be used to improve the affinity andspecificity of the monobody. Purified monobody samples are dissolved inan appropriate buffer for NMR spectroscopy using Amicon® ultrafiltrationcell with a YM-3 membrane. Buffers are made with 90% H₂O/10% D₂O(distilled grade, Isotec) or with 100% D₂O. Deuterated compounds (e.g.acetate) are used to eliminate strong signals from them.

NMR experiments are performed on a Varian Unity INOVA 600 spectrometerequipped with four RF channels and a triple resonance probe with pulsedfield gradient capability. NMR spectra are analyzed using processingprograms such as Felix (Molecular Simulations), nmrPipe, PIPP, and CAPP(Garrett, et al., 1991; Delaglio, et al., 1995) on UNIX workstations.Sequence specific resonance assignments are made using well-establishedstrategy using a set of triple resonance experiments (CBCA(CO)NH andHNCACB) (Grzesiek & Bax, 1992; Wittenkind & Mueller, 1993).

Nuclear Overhauser effect (NOE) is observed between ¹H nuclei closerthan approximately 5 Å, which allows one to obtain information oninterproton distances. A series of double- and triple-resonanceexperiments (Table 6; for recent reviews on these techniques, see Bax &Grzesiek, 1993 and Kay, 1995) are performed to collect distance (i.e.NOE) and dihedral angle (J-coupling) constraints. Isotope-filteredexperiments are performed to determine resonance assignments of thebound ligand and to obtain distance constraints within the ligand andthose between FnAb and the ligand. Details of sequence specificresonance assignments and NOE peak assignments have been described indetail elsewhere (Clore & Gronenborn, 1991; Pascal, et al., 1994b;Metzler, et al., 1996).

TABLE 6 NMR experiments for structure characterization Experiment NameReference 1. reference spectra 2D-¹H, ¹⁵N-HSQC (Bodenhausen & Ruben,1980; Kay, et al., 1992) 2D-¹H, ¹³C-HSQC (Bodenhausen & Ruben, 1980;Vuister & Bax, 1992) 2. backbone and side chain resonance assignments of¹³C/¹⁵N-labeled protein 3D-CBCA(CO)NH (Grzesiek & Bax, 1992) 3D-HNCACB(Wittenkind & Mueller, 1993) 3D-C(CO)NH (Logan et al., 1992; Grzesiek etal., 1993) 3D-H(CCO)NH 3D-HBHA(CBCACO)NH (Grzesiek & Bax, 1993)3D-HCCH-TOCSY (Kay et al., 1993) 3D-HCCH-COSY (Ikura et al., 1991)3D-¹H, ¹⁵N-TOCSY-HSQC (Zhang et al., 1994) 2D-HB(CBCDCE)HE (Yamazaki etal., 1993) 3. resonance assignments of unlabeled ligand2D-isotope-filtered ¹H-TOCSY 2D-isotope-filtered ¹H-COSY2D-isotope-filtered ¹H-NOESY (Ikura & Bax, 1992) 4. structuralconstraints within labeled protein 3D-¹H, ¹⁵N-NOESY-HSQC (Zhang et al.,1994) 4D-¹H, ¹³C-HEMQC-NOESY-HMQC (Vuister et al., 1993) 4D-¹H, ¹³C,¹⁵N-HQC-NOESY-HSQC (Muhandiram et al., 1993; Pascal et al., 1994a)within unlabeled ligand 2D-isotope-filtered ¹H-NOESY (Ikura & Bax, 1992)interactions between protein and ligand 3D-isotope-filtered ¹H,¹⁵N-NOESY-HSQC 3D-isotope-filtered ¹H, ¹³C-NOESY-HSQC (Lee et al., 1994)5. dihedral angle constraints J-molulated ¹H, ¹⁵N-HSQC (Billeter et al.,1992) 3D-HNHB (Archer et al., 1991)

Backbone ¹H, ¹⁵N and ¹³C resonance assignments for a monobody arecompared to those for wild-type Fn3 to assess structural changes in themutant. Once these data establish that the mutant retains the globalstructure, structural refinement is performed using experimental NOEdata. Because the structural difference of a monobody is expected to beminor, the wild-type structure can be used as the initial model aftermodifying the amino acid sequence. The mutations are introduced to thewild-type structure by interactive molecular modeling, and then thestructure is energy-minimized using a molecular modeling program such asQuanta (Molecular Simulations). Solution structure is refined usingcycles of dynamical simulated annealing (Nilges et al., 1988) in theprogram X-PLOR (Brünger, 1992). Typically, an ensemble of fiftystructures is calculated. The validity of the refined structures isconfirmed by calculating a fewer number of structures from randomlygenerated initial structures in X-PLOR using the YASAP protocol (Nilges,et al., 1991). Structure of a monobody-ligand complex is calculated byfirst refining both components individually using intramolecular NOEs,and then docking the two using intermolecular NOEs.

For example, the ¹H, ¹⁵N-HSQC spectrum for the fluorescein-bindingmonobody LB25.5 is shown in FIG. 14. The spectrum shows a gooddispersion (peaks are spread out) indicating that LB25.5 is folded intoa globular conformation. Further, the spectrum resembles that for thewild-type Fn3, showing that the overall structure of LB25.5 is similarto that of Fn3. These results demonstrate that ligand-binding monobodiescan be obtained without changing the global fold of the Fn3 scaffold.

Chemical shift perturbation experiments are performed by forming thecomplex between an isotope-labeled FnAb and an unlabeled ligand. Theformation of a stoichiometric complex is followed by recording the HSQCspectrum. Because chemical shift is extremely sensitive to nuclearenvironment, formation of a complex usually results in substantialchemical shift changes for resonances of amino acid residues in theinterface. Isotope-edited NMR experiments (2D HSQC and 3D CBCA(CO)NH)are used to identify the resonances that are perturbed in the labeledcomponent of the complex; i.e. the monobody. Although the possibility ofartifacts due to long-range conformational changes must always beconsidered, substantial differences for residues clustered on continuoussurfaces are most likely to arise from direct contacts (Chen et al.,1993; Gronenbom & Clore, 1993).

An alternative method for mapping the interaction surface utilizes amidehydrogen exchange (HX) measurements. HX rates for each amide proton aremeasured for ¹⁵N labeled monobody both free and complexed with a ligand.Ligand binding is expected to result in decreased amide HX rates formonobody residues in the interface between the two proteins, thusidentifying the binding surface. HX rates for monobodies in the complexare measured by allowing FIX to occur for a variable time followingtransfer of the complex to D₂O; the complex is dissociated by loweringpH and the HSQC spectrum is recorded at low pH where amide HX is slow.Fn3 is stable and soluble at low pH, satisfying the prerequisite for theexperiments.

Example XVI Construction and Analysis of Fn3-Display System Specific forUbiquitin

An Fn3-display system was designed and synthesized, ubiquitin-bindingclones were isolated and a major Fn3 mutant in these clones wasbiophysically characterized.

Gene construction and phage display of Fn3 was performed as in ExamplesI and II above. The Fn3-phage pIII fusion protein was expressed from aphagemid-display vector, while the other components of the M13 phage,including the wild-type pIII, were produced using a helper phage (Basset al., 1990). Thus, a phage produced by this system should contain lessthan one copy of Fn3 displayed on the surface. The surface display ofFn3 on the phage was detected by ELISA using an anti-Fn3 antibody. Onlyphages containing the Fn3-pIII fusion vector reacted with the antibody.

After confirming the phage surface to display Fn3, a phage displaylibrary of Fn3 was constructed as in Example III. Random sequences wereintroduced in the BC and FG loops. In the first library, five residues(77-81) were randomized and three residues (82-84) were deleted from theFG loop. The deletion was intended to reduce the flexibility and improvethe binding affinity of the FG loop. Five residues (26-30) were alsorandomized in the BC loop in order to provide a larger contact surfacewith the target molecule. Thus, the resulting library contains fiverandomized residues in each of the BC and FG loops (Table 7). Thislibrary contained approximately 10⁸ independent clones.

Library Screening

Library screening was performed using ubiquitin as the target molecule.In each round of panning, Fn3-phages were absorbed to a ubiquitin-coatedsurface, and bound phages were eluted competitively with solubleubiquitin. The recovery ratio improved from 4.3×10⁻⁷ in the second roundto 4.5×10⁻⁶ in the fifth round, suggesting an enrichment of bindingclones. After five founds of panning, the amino acid sequences ofindividual clones were determined (Table 7).

TABLE 7 Sequences in the variegated loops of enriched clones NameBC loop FG loop Frequency Wild GCAGTTACCGTGCGT GGCCGTGGTGACAGCCCAGCGAGC— Type (SEQ ID NO: 93) (SEQ ID NO: 95) AlaValThrValArgGlyArgGlyAspSerProAlaSer (SEQ ID NO: 94) (SEQ ID NO: 96) Library^(a)NNKNNKNNKNNKNNK NNKNNKNNKNNKNNK--------- — X X X X XX X X X X (deletion) clone1 TCGAGGTTGCGGCGG CCGCCGTGGAGGGTG 9 (Ubi4)(SEQ ID NO: 97) (SEQ ID NO: 99) SerArgLeuArgArg ProProTrpArgVal(SEQ ID NO: 98) (SEQ ID NO: 100) clone2 GGTCAGCGAACTTTT AGGCGGTGGTGGGCT1 (SEQ ID NO: 101) (SEQ ID NO: 103) GlyGlnArgThrPhe ArgArgTrpTrpAla(SEQ ID NO: 102) (SEQ ID NO: 104) clone3 GCGAGGTGGACGCTT AGGCGGTGGTGGTGG1 (SEQ ID NO: 105) (SEQ ID NO: 107) AlaArgTrpThrLeu ArgArgTrpTrpTrp(SEQ ID NO: 106) (SEQ ID NO: 108) ^(a)N denotes an equimolar mixture ofA, T, G and C; K denotes an equimolar mixture of G and T.A clone, dubbed Ubi4, dominated the enriched pool of Fn3 variants.Therefore, further investigation was focused on this Ubi4 clone. Ubi4contains four mutations in the BC loop (Arg 30 in the BC loop wasconserved) and five mutations and three deletions in the FG loop. Thus13% (12 out of 94) of the residues were altered in Ubi4 from thewild-type sequence.

FIG. 15 shows a phage ELISA analysis of Ubi4. The Ubi4 phage binds tothe target molecule, ubiquitin, with a significant affinity, while aphage displaying the wild-type Fn3 domain or a phase with no displayedmolecules show little detectable binding to ubiquitin (FIG. 15 a). Inaddition, the Ubi4 phage showed a somewhat elevated level of backgroundbinding to the control surface lacking the ubiquitin coating. Acompetition ELISA experiments shows the IC₅₀ (concentration of the freeligand which causes 50% inhibition of binding) of the binding reactionis approximately 5 μM (FIG. 15 b). BSA, bovine ribonuclease A andcytochrome C show little inhibition of the Ubi4-ubiquitin bindingreaction (FIG. 15 c), indicating that the binding reaction of Ubi4 toubiquitin does result from specific binding.

Characterization of a Mutant Fn3 Protein

The expression system yielded 50-100 mg Fn3 protein per liter culture. Asimilar level of protein expression was observed for the Ubi4 clone andother mutant Fn3 proteins.

Ubi4-Fn3 was expressed as an independent protein. Though a majority ofUbi4 was expressed in E. coli as a soluble protein, its solubility wasfound to be significantly reduced as compared to that of wild-type Fn3.Ubi4 was soluble up to ˜20 μM at low pH, with much lower solubility atneutral pH. This solubility was not high enough for detailed structuralcharacterization using NMR spectroscopy or X-ray crystallography.

The solubility of the Ubi4 protein was improved by adding a solubilitytail, GKKGK (SEQ ID NO:109), as a C-terminal extension. The gene forUbi4-Fn3 was subcloned into the expression vector pAS45 using PCR. TheC-terminal solubilization tag, GKKGK (SEQ ID NO:109), was incorporatedin this step. E. coli BL21 (DE3) (Novagen) was transformed with theexpression vector (pAS45 and its derivatives). Cells were grown in M9minimal media and M9 media supplemented with Bactotryptone (Difco)containing ampicillin (200 μg/ml). For isotopic labeling, ¹⁵N NH₄Clreplaced unlabeled NH₄Cl in the media. 500 ml medium in a 2 liter baffleflask was inoculated with 10 ml of overnight culture and agitated at 37°C. IPTG was added at a final concentration of 1 mM to initiate proteinexpression when OD (600 nm) reaches one. The cells were harvested bycentrifugation 3 hours after the addition of IPTG and kept frozen at−70° C. until used.

Proteins were purified as follows. Cells were suspended in 5 ml/(g cell)of Tris (50 mM, pH 7.6) containing phenylmethylsulfonyl fluoride (1 mM).Hen egg lysozyme (Sigma) was added to a final concentration of 0.5mg/ml. After incubating the solution for 30 minutes at 37° C., it wassonicated three times for 30 seconds on ice. Cell debris was removed bycentrifugation. Concentrated sodium chloride was added to the solutionto a final concentration of 0.5 M. The solution was applied to aHi-Trap™ chelating column (Pharmacia) preloaded with nickel andequilibrated in the Tris buffer containing sodium chloride (0.5 M).After washing the column with the buffer, histag-Fn3 was eluted with thebuffer containing 500 mM imidazole. The protein was further purifiedusing a ResourceS column (Pharmacia) with a NaCl gradient in a sodiumacetate buffer (20 mM, pH 4.6).

With the GKKGK (SEQ ID NO:109) tail, the solubility of the Ubi4 proteinwas increased to over 1 mM at low pH and up to ˜50 μM at neutral pH.Therefore, further analyses were performed on Ubi4 with this C-terminalextension (hereafter referred to as Ubi4-K). It has been reported thatthe solubility of a minibody could be significantly improved by additionof three Lys residues at the—or C-termini (Bianchi et al., 1994). In thecase of protein Rop, a non-structured C-terminal tail is critical inmaintaining its solubility (Smith et al., 1995).

Oligomerization states of the Ubi4 protein were determined using a sizeexclusion column. The wild-type Fn3 protein was monomeric at low andneutral pH's. However, the peak of the Ubi4-K protein was significantlybroader than that of wild-type Fn3, and eluted after the wild-typeprotein. This suggests interactions between Ubi4-K and the columnmaterial, precluding the use of size exclusion chromatography todetermine the oligomerization state of Ubi4. NMR studies suggest thatthe protein is monomeric at low pH.

The Ubi4-K protein retained a binding affinity to ubiquitin as judged byELISA (FIG. 15 d). However, an attempt to determine the dissociationconstant using a biosensor (Affinity Sensors, Cambridge, U.K.) failedbecause of high background binding of Ubi4-K-Fn3 to the sensor matrix.This matrix mainly consists of dextran, consistent with the observationthat interactions between Ubi4-K interacts with the cross-linked dextranof the size exclusion column.

Example XVII Stability Measurements of Monobodies

Guanidine hydrochloride (GuHCl)-induced unfolding and refoldingreactions were followed by measuring tryptophan fluorescence.Experiments were performed on a Spectronic AB-2 spectrofluorometerequipped with a motor-driven syringe (Hamilton Co.). The cuvettetemperature was kept at 30° C. The spectrofluorometer and the syringewere controlled by a single computer using a home-built interface. Thissystem automatically records a series of spectra following GuHCltitration. An experiment started with a 1.5 ml buffer solutioncontaining 5 μM protein. An emission spectrum (300-400 nm; excitation at290 nm) was recorded following a delay (3-5 minutes) after eachinjection (50 or 100 μl) of a buffer solution containing GuHCl. Thesesteps were repeated until the solution volume reached the full capacityof a cuvette (3.0 ml). Fluorescence intensities were normalized asratios to the intensity at an isofluorescent point which was determinedin separate experiments. Unfolding curves were fitted with a two-statemodel using a nonlinear least-squares routine (Santoro & Bolen, 1988).No significant differences were observed between experiments with delaytimes (between an injection and the start of spectrum acquisition) of 2minutes and 10 minutes, indicating that the unfolding/refoldingreactions reached close to an equilibrium at each concentration pointwithin the delay times used.

Conformational stability of Ubi4-K was measured using above-describedGuHCl-induced unfolding method. The measurements were performed undertwo sets of conditions; first at pH 3.3 in the presence of 300 mM sodiumchloride, where Ubi4-K is highly soluble, and second in TBS, which wasused for library screening. Under both conditions, the unfoldingreaction was reversible, and we detected no signs of aggregation orirreversible unfolding. FIG. 16 shows unfolding transitions of Ubi4-Kand wild-type Fn3 with the N-terminal (his)₆ tag and the C-terminalsolubility tag. The stability of wild-type Fn3 was not significantlyaffected by the addition of these tags. Parameters characterizing theunfolding transitions are listed in Table 8.

TABLE 8 Stability parameters for Ubi4 and wild-type Fn3 as determined byGuHCl-induced unfolding Protein ΔG₀ (kcal mol⁻¹) m_(G) (kcal mol⁻¹ M⁻¹)Ubi4 (pH 7.5) 4.8 ± 0.1 2.12 ± 0.04 Ubi4 (pH 3.3) 6.5 ± 0.1 2.07 ± 0.02Wild-type (pH 7.5) 7.2 ± 0.2 1.60 ± 0.04 Wild-type (pH 3.3) 11.2 ± 0.1 2.03 ± 0.02 ΔG₀ is the free energy of unfolding in the absence ofdenaturant; m_(G) is the dependence of the free energy of unfolding onGuHCl concentration. For solution conditions, see FIG. 4 caption.Though the introduced mutations in the two loops certainly decreased thestability of Ubi4-K relative to wild-type Fn3, the stability of Ubi4remains comparable to that of a “typical” globular protein. It shouldalso be noted that the stabilities of the wild-type and Ubi4-K proteinswere higher at pH 13 than at pH 7.5.

The Ubi4 protein had a significantly reduced solubility as compared tothat of wild-type Fn3, but the solubility was improved by the additionof a solubility tail. Since the two mutated loops include the onlydifferences between the wild-type and Ubi4 proteins, these loops must bethe origin of the reduced solubility. At this point, it is not clearwhether the aggregation of Ubi4-K is caused by interactions between theloops, or by interactions between the loops and the invariable regionsof the Fn3 scaffold.

The Ubi4-K protein retained the global fold of Fn3, showing that thisscaffold can accommodate a large number of mutations in the two loopstested. Though the stability of the Ubi4-K protein is significantlylower than that of the wild-type Fn3 protein, the Ubi4 protein still hasa conformational stability comparable to those for small globularproteins. The use of a highly stable domain as a scaffold is clearlyadvantageous for introducing mutations without affecting the global foldof the scaffold. In addition, the GuHCl-induced unfolding of the Ubi4protein is almost completely reversible. This allows the preparation ofa correctly folded protein even when a Fn3 mutant is expressed in amisfolded form, as in inclusion bodies. The modest stability of Ubi4 inthe conditions used for library screening indicates that Fn3 variantsare folded on the phage surface. This suggests that a Fn3 clone isselected by its binding affinity in the folded form, not in a denaturedform. Dickinson et al. proposed that Val 29 and Arg 30 in the BC loopstabilize Fn3. Val 29 makes contact with the hydrophobic core, and Arg30 forms hydrogen bonds with Gly 52 and Val 75. In Ubi4-Fn3, Val 29 isreplaced with Mg, while Arg 30 is conserved. The FG loop was alsomutated in the library. This loop is flexible in the wild-typestructure, and shows a large variation in length among human Fn3 domains(Main et al., 1992). These observations suggest that mutations in the FGloop may have less impact on stability. In addition, the N-terminal tailof Fn3 is adjacent to the molecular surface formed by the BC and FGloops (FIGS. 1 and 17) and does not form a well-defined structure.Mutations in the N-terminal tail would not be expected to have strongdetrimental effects on stability. Thus, residues in the N-terminal tailmay be good sites for introducing additional mutations.

Example XVIII NMR Spectroscopy of Ubi4-Fn3

Ubi4-Fn3 was dissolved in HCl buffer (20 mM, pH 3.3) containing NaCl(300 mM) using an Amicon ultrafiltration unit. The final proteinconcentration was 1 mM. NMR experiments were performed on a Varian UnityINOVA 600 spectrometer equipped with a triple-resonance probe withpulsed field gradient. The probe temperature was set at 30° C. HSQC,TOCSY-HSQC and NOESY-HSQC spectra were recorded using publishedprocedures (Kay et al., 1992; Zhang et al., 1994). NMR spectra wereprocessed and analyzed using the NMRPipe and NMRView software (Johnson &Blevins, 1994; Delaglio et al., 1995) on UNIX workstations.Sequence-specific resonance assignments were made using standardprocedures (Wüthrich, 1986; Clore & Gronenbom, 1991). The assignmentsfor wild-type Fn3 (Baron et al., 1992) were confirmed using a¹⁵N-labeled protein dissolved in sodium acetate buffer (50 mM, pH 4.6)at 30° C.

The three-dimensional structure of Ubi4-K was characterized using thisheteronuclear NMR spectroscopy method. A high quality spectrum could becollected on a 1 mM solution of ¹⁵N-labeled Ubi4 (FIG. 17 a) at low pH.The linewidth of amide peaks of Ubi4-K was similar to that of wild-typeFn3, suggesting that Ubi4-K is monomeric under the conditions used.Complete assignments for backbone ¹H and ¹⁵N nuclei were achieved usingstandard ¹H, ¹⁵N double resonance techniques, except for a row of Hisresidues in the N-terminal (His)₆ tag. There were a few weak peaks inthe HSQC spectrum which appeared to originate from a minor speciescontaining the N-terminal Met residue. Mass spectroscopy analysis showedthat a majority of Ubi4-K does not contain the N-terminal Met residue.FIG. 17 shows differences in ¹HN and ¹⁵N chemical shifts between Ubi4-Kand wild-type Fn3. Only small differences are observed in the chemicalshifts, except for those in and near the mutated BC and FG loops. Theseresults clearly indicate that Ubi4-K retains the global fold of Fn3,despite the extensive mutations in the two loops. A few residues in theN-terminal region, which is close to the two mutated loops, also exhibitsignificant chemical differences between the two proteins. An HSQCspectrum was also recorded on a 50 μM sample of Ubi4-K in TBS. Thespectrum was similar to that collected at low pH, indicating that theglobal conformation of Ubi4 is maintained between pH 7.5 and 3.3.

Example XIX Stabilization of Fn3 Domain by Removing UnfavorableElectrostatic Interactions on the Protein Surface Introduction

Increasing the conformational stability of a protein by mutation is amajor interest in protein design and biotechnology. Thethree-dimensional structures of proteins are stabilized by combinationof different types of forces. The hydrophobic effect, van der Waalsinteractions and hydrogen bonds are known to contribute to stabilize thefolded state of proteins (Kauzmann, W. (1959) Adv. Prot. Chem. 14, 1-63;Dill, K. A. (1990) Biochemistry 29, 7133-7155; Pace, C. N., Shirley, B.A., McNutt, M. & Gajiwala, K. (1996) Faseb J 10, 75-83). Thesestabilizing forces primarily originate from residues that are wellpacked in a protein, such as those that constitute the hydrophobic core.Because a change in the protein core would induce a rearrangement ofadjacent moieties, it is difficult to improve protein stability byincreasing these forces without massive computation (Malakauskas, S. M.& Mayo, S. L. (1998) Nat Struct Biol 5, 470-475). Ion pairs betweencharged groups are commonly found on the protein surface (Creighton, T.E. (1993) Proteins: structures and molecular properties, Freeman, NewYork), and an ion pair could be introduced to a protein with smallstructural perturbations. However, a number of studies have demonstratedthat the introduction of an attractive electrostatic interaction, suchas an ion pair, on protein surface has small effects on stability(Dao-pin, S., Sauer, U., Nicholson, H. & Matthews, B. W. (1991)Biochemistry 30, 7142-7153; Sali, D., Bycroft, M. & Fersht, A. R. (1991)J. Mol. Biol. 220, 779-788). A large desolvation penalty and the loss ofconformational entropy of amino acid side chains oppose the favorableelectrostatic contribution (Yang, A.-S. & Honig, B. (1992) Curr. Opin.Struct. Biol. 2, 40-45; Hendsch, Z. S. & Tidor, B. (1994) Protein Sci.3, 211-226). Recent studies demonstrated that repulsive electrostaticinteractions on the protein surface, in contrast, may significantlydestabilize a protein, and that it is possible to improve proteinstability by optimizing surface electrostatic interactions (Loladze, V.V., Ibarra-Molero, B., Sanchez-Ruiz, J. M. & Makhatadze, G. I. (1999)Biochemistry 38, 16419-16423; Perl, D., Mueller, U., Heinemann, U. &Schmid, F. X. (2000) Nat Struct Biol 7, 380-383; Spector, S., Wang, M.,Carp, S. A., Robblee, J., Hendsch, Z. S., Fairman, R., Tidor, B. &Raleigh, D. P. (2000) Biochemistry 39, 872-879; Grimsley, G. R., Shaw,K. L., Fee, L. R., Alston, R. W., Huyghues-Despointes, B. M., Thurikill,R. L., Scholtz, J. M. & Pace, C. N. (1999) Protein Sci 8, 1843-1849). Inthe present experiments, the inventor improved protein stability bymodifying surface electrostatic interactions.

During the characterization of monobodies it was found that theseproteins, as well as wild-type FNfn10, are significantly more stable atlow pH than at neutral pH (Koide, A., Bailey, C. W., Huang, X. & Koide,S. (1998) J. Mol. Biol. 284, 1141-1151). These observations indicatethat changes in the ionization state of some moieties in FNfn10 modulatethe conformational stability of the protein, and suggest that it mightbe possible to enhance the conformational stability of FNfn10 at neutralpH by adjusting electrostatic properties of the protein. Improving theconformational stability of FNfn10 will also have practical importancein the use of FNfn10 as a scaffold in biotechnology applications.

Described below are experiments that detailed characterization of the pHdependence of FNfn10 stability, identified unfavorable interactionsbetween side chain carboxyl groups, and improved the conformationalstability of FNfn10 by point mutations on the surface. The resultsdemonstrate that the surface electrostatic interactions contributesignificantly to protein stability, and that it is possible to enhanceprotein stability by rationally modulating these interactions.

Experimental Procedures

Protein Expression and Purification

The wild-type protein used for the NMR studies contained residues 1-94of FNfn10 (residue numbering is according to FIG. 2( a) of Koide et al.(Koide, A., Bailey, C. W., Huang, X. & Koide, S. (1998) J. Mol. Biol.284, 1141-1151)), and additional two residues (Met-Gln) at theN-terminus (these two residues are numbered −2 and −1, respectively).The gene coding for the protein was inserted in pET3a (Novagen, Wis.).Eschericha coli BL21 (DE3) transformed with the expression vector wasgrown in the M9 minimal media supplemented with ¹³C-glucose and¹⁵N-ammonium chloride (Cambridge Isotopes) as the sole carbon andnitrogen sources, respectively. Protein expression was induced asdescribed previously (Koide, A., Bailey, C. W., Huang, X. & Koide, S.(1998) J. Mol. Biol. 284, 1141-1151). After harvesting the cells bycentrifuge, the cells were lysed as described (Koide, A., Bailey, C. W.,Huang, X. & Koide, S. (1998) J. Mol. Biol. 284, 1141-1151). Aftercentrifugation, supernatant was dialyzed against 10 mM sodium acetatebuffer (pH 5.0), and the protein solution was applied to a SP-Sepharose®FastFlow column (Amersham Pharmacia Biotech), and FN3 was eluted with agradient of sodium chloride. The protein was concentrated using anAmicon® concentrator using YM-3 membrane (Millipore).

The wild-type protein used for the stability measurements contained anN-terminal histag (MGSSHHHHHHSSGLVPRGSH) (SEQ ID NO:114) and residues−2-94 of FNfn10. The gene for FN3 described above was inserted in pET15b(Novagen). The protein was expressed and purified as described (Koide,A., Bailey, C. W., Huang, X. & Koide, S. (1998) J. Mol. Biol. 284,1141-1151). The wild-type protein used for measurements of the pHdependence shown in FIG. 22 contained Arg 6 to Thr mutation, which hadoriginally been introduced to remove a secondary thrombin cleavage site(Koide, A., Bailey, C. W., Huang, X. & Koide, S. (1998) J. Mol. Biol.284, 1141-1151). Because Asp 7, which is adjacent to Arg 6, was found tobe critical in the pH dependence of FN3 stability as detailed underResults, subsequent studies were performed using the wild-type, Arg 6,background. The genes for the D7N and D7K mutants were constructed usingstandard polymerase chain reactions, and inserted in pET15b. Theseproteins were prepared in the same manner as for the wild-type protein.¹³C, ¹⁵N-labeled proteins for pK_(a) measurements were prepared asdescribed above, and the histag moiety was not removed from theseproteins.

Chemical Denaturation Measurements

Proteins were dissolved to a final concentration of 5 μM in 10 mM sodiumcitrate buffer at various pH containing 100 mM sodium chloride.Guanidine HCl (GuHCl)-induce unfolding experiments were performed asdescribed previously (Koide, A., Bailey, C. W., Huang, X. & Koide, S.(1998) J. Mol. Biol. 284, 1141-1151; Koide, S., Bu, Z., Risal, D., Pham,T.-N., Nakagawa, T., Tamura, A. & Engelman, D. M. (1999) Biochemistry38, 4757-4767). GuHCl concentration was determined using an Abberefractometer (Spectronic Instruments) as described (Pace, C. N. &Sholtz, J. M. (1997) in Protein structure. A practical approach(Creighton, T. E., Ed.) Vol. pp 299-321, IRL Press, Oxford). Data wereanalyzed according to the two-state model as described (Koide, A.,Bailey, C. W., Huang, X & Koide, S. (1998) J. Mol. Biol. 284, 1141-1151;Santoro, M. M. & Bolen, D. W. (1988) Biochemistry 27, 8063-8068.).

Thermal Denaturation Measurements

Proteins were dissolved to a final concentration of 5 μM in 20 mM sodiumphosphate buffer (pH 7.0) containing 0.1 or 1 M sodium chloride or in 20mM glycine HCl buffer (pH 2.4) containing 0.1 or 1 M sodium chloride.Additionally 6.3 M urea was included in all solutions to ensurereversibility of the thermal denaturation reaction. In the absence ofurea it was found that denatured FNfn10 adheres to quartz surface, andthat the thermal denaturation reaction was irreversible. Circulardichroism measurements were performed using a Model 202 spectrometerequipped with a Peltier temperature controller (Aviv Instruments). Acuvette with a 0.5-cm pathlength was used. The ellipticity at 227 nm wasrecorded as the sample temperature was raised at a rate of approximately1° C. per minute. Because of decomposition of urea at high temperature,the pH of protein solutions tended to shift upward during an experiment.The pH of protein solution was measured before and after each thermaldenaturation measurement to ensure that a shift no more than 0.2 pH unitoccurred in each measurement. At pH 2.4, two sections of a thermaldenaturation curve (30-65° C. and 60-95° C.) were acquired from separatesamples, in order to avoid a large pH shift. The thermal denaturationdata were fit with the standard two-state model (Pace, C. N. & Sholtz,J. M. (1997) in Protein structure. A practical approach (Creighton, T.E., Ed.) Vol. pp 299-321, IRL Press, Oxford):ΔG(T)=ΔH _(m)(1−T/T _(m))−ΔC _(p)[(T _(m) −T)+T ln(T/T _(m))]where ΔG(T) is the Gibbs free energy of unfolding at temperature T,ΔH_(m) is the enthalpy change upon unfolding at the midpoint of thetransition, T_(m), and ΔC_(P) is the heat capacity change uponunfolding. The value for ΔC_(p) was fixed at 1.74 kcal mol⁻¹ K⁻¹,according to the approximation of Myers et al. (Myers, J. K., Pace, C.N. & Scholtz, J. M. (1995) Protein Sci. 4, 2138-2148). Most of thedatasets taken in the presence of 1 M NaCl did not have a sufficientbaseline for the unfolded state, and thus it was assumed the slope ofthe unfolded baseline in the presence of 1 M NaCl to be identical tothat determined in the presence of 0.1 M NaCl.NMR Spectroscopy

NMR experiments were performed at 30° C. on an INOVA 600 spectrometer(Varian Instruments). The C(CO)NH experiment (Grzesiek, S., Anglister,J. & Bax, A. (1993) J. Magn. Reson. B 101, 114-119) and the CBCACOHAexperiment (Kay, L. E. (1993) J. Am. Chem. Soc. 115, 2055-2057) werecollected on a [¹³C, ¹⁵N]-wild-type FNfn10 sample (1 mM) dissolved in 50mM sodium acetate buffer (pH 4.6) containing 5% (v/v) deuterium oxide,using a Varian 5 mm triple resonance probe with pulsed field gradient.The carboxyl ¹³C resonances were assigned based on the backbone ¹H, ¹³Cand ¹⁵N resonance assignments of FNfn10 (Baron, M., Main, A. L.,Driscoll, P. C., Mardon, H. J., Boyd, J. & Campbell, I. D. (1992)Biochemistry 31, 2068-2073). pH titration of carboxyl resonances wereperformed on a 0.3 mM FNfn10 sample dissolved in 10 mM sodium citratecontaining 100 mM sodium chloride and 5% (v/v) deuterium oxide. An 8 mmtriple-resonance, pulse-field gradient probe (Nanolac Corporation) wasused for pH titration. Two-dimensional H(C)CO spectra were collectedusing the CBCACOHA pulse sequence as described previously (McIntosh, L.P., Hand, G., Johnson, P. E., Joshi, M. D., Koerner, M., Plesniak, L.A., Ziser, L., Wakarchuk, W. W. & Withers, S. G. (1996) Biochemistry 35,9958-9966). Sample pH was changed by adding small aliquots ofhydrochloric acid, and pH was measured before and after taking NMR data.¹H, ¹⁵N-HSQC spectra were taken as described previously (Kay, L. E.,Keifer, P. & Saarinen, T. (1992) J. Am. Chem. Soc. 114, 10663-10665).NMR data were processed using the NMRPipe package (Delaglio, F.,Grzesiek, S., Vuister, G. W., Zhu, G., Pfeifer, J. & Bax, A. (1995) J.Biomol. NMR 6, 277-293), and analyzed using the NMRView software(Johnson, B. A. & Blevins, R. A. (1994) J. Biomol. NMR 4, 603-614).

NMR titration curves of the carboxyl ¹³C resonances were fit to theHenderson-Hasselbalch equation to determine pK_(a)'s:δ(pH)=(δ_(acid)+δ_(base)10^((pH−pK) ^(a) ⁾)/(1+10^((pH−pKa)))where S is the measured chemical shift, δ_(acid) is the chemical shiftassociated with the protonated state, δ_(base) is the chemical shiftassociated with the deprotonated state, and pK_(a) is the pK_(a) valuefor the residue. Data were also fit to an equation with two ionizablegroups:δ(pH)=(δ_(AH2′)+δ_(AH)10^((pH−pK) ^(a1) ⁾+δ_(A)10^((2pH−pK) ^(a1) ^(−pK)^(a2) ⁾)/(1+10^((pH−pK) ^(a1) ⁾+10^((2pH−pK) ^(a1) ^(−pK) _(a2) ⁾)where δ_(AH2), δ_(AH) and δ_(A) are the chemical shifts associated withthe fully protonated, singularly protonated and deprotonated states,respectively, and pK_(a1) and pK_(a2) are pK_(a)'s associated with thetwo ionization steps. Data fitting was performed using the nonlinearleast-square regression method in the program Igor Pro (WaveMetrix, OR)on a Macintosh computer.ResultspH Dependence of FNfn10 Stability

Previously, it was found that FNfn10 is more stable at acidic pH than atneutral pH (Koide, A., Bailey, C. W., Huang, X. & Koide, S. (1998) J.Mol. Biol. 284, 1141-1151). In the present experiments, the pHdependence of its stability was further characterized. Because of itshigh stability, FNfn10 could not be fully denatured in urea at 30° C.Thus GuHCl-induced chemical denaturation (FIG. 18) was used. Thedenaturation reaction was fully reversible under all conditions tested.In order to minimize errors caused by extrapolation, the free energy ofunfolding at 4 M GuHCl was used for comparison (FIG. 18). The stabilityincreased as the pH was lowered, with apparent plateaus at both ends ofthe pH range. The pH dependence curve has an apparent transitionmidpoint near pH 4. In addition, a gradual increase in the m value, thedependence of the unfolding free energy on denaturant concentration wasnoted. Pace et al. reported a similar pH dependence of the m value forbarnase (Pace, C. N., Laurents, D. V. & Erickson, R. E. (1992)Biochemistry 31, 2728-2734). These results indicate that FNfn10 containsinteractions that stabilize the protein at low pH, or those thatdestabilize it at neutral pH. The results also suggest that byidentifying and altering the interactions that give rise to the pHdependence, one may be able to improve the stability of FNfn10 atneutral pH to a degree similar to that found at low pH.

Determination of pK_(a)'s of the Side Chain Carboxyl Groups in Wild-TypeFNfn10

The pH dependence of FNfn10 stability suggests that amino acids withpK_(a) near 4 are involved in the observed transition. The carboxylgroups of Asp and Glu generally have pK_(a) in this range (Creighton, T.E. (1993) Proteins: structures and molecular properties, Freeman, NewYork). It is well known that if a carboxyl group has unfavorable (i.e.destabilizing) interactions in the folded state, its pK_(a) is shiftedto a higher value from its unperturbed value (Yang, A.-S. & Honig, B.(1992) Curr. Opin. Struct. Biol. 2, 40-45). If a carboxyl group hasfavorable interactions in the folded state, it has a lower pK_(a). Thus,the pK_(a), values of all carboxylates in FNfn10 using heteronuclear NMRspectroscopy were determined in order to identify stabilizing anddestabilizing interactions involving carboxyl groups.

First, the ¹³C resonance for the carboxyl carbon of each Asp and Gluresidue in FN3 was assigned (FIG. 19). Next, pH titration of the ¹³Cresonances for these groups was performed (FIG. 20). Titration curvesfor Asp 3, 67 and 80, and Glu 38 and 47 could be fit well with theHenderson-Hasselbalch equation with a single pK_(a). The pK_(a) valuesfor these residues (Table 9) are either close to or slightly lower thantheir respective unperturbed values (3.8-4.1 for Asp, and 4.1-4.6 forGlu (Kuhlman, B., Luisi, D. L., Young, P. & Raleigh, D. P. (1999)Biochemistry 38, 4896-4903)), indicating that these carboxyl groups areinvolved in neutral or slightly favorable electrostatic interactions inthe folded state.

TABLE 9 pK_(a) values for Asp and Glu residues in FN3¹. Protein ResidueWild-Type D7N D7K E9 3.84, 5.40² 4.98 4.53 E38 3.79 3.87 3.86 E47 3.943.99 3.99 D3 3.66 3.72 3.74 D7 3.54, 5.54² — — D23 3.54, 5.25² 3.68 3.82D67 4.18 4.17 4.14 D80 3.40 3.49 3.48 ¹Th e standard deviations in thepK_(a) values are less than 0.05 pH units for those fit with a singlepK_(a) and less than 0.15 pH unit for those with two pK_(a)'s. ²Data forE9, D7 and D23 were fit with a transition curve with two pK_(a) values.

The titration curves for Asp 7 and 23, and Glu 9 were fit better withthe Henderson-Hasselbalch equation with two pK_(a) values, and one ofthe two pK_(a) values for each were shifted higher than the respectiveunperturbed values (FIG. 19B). The titration curves with two apparentpK_(a) values of these carboxyl groups may be due to influence of anionizable group in the vicinity. In the three-dimensional structure ofFNfn10 (Main, A. L., Harvey, T. S., Baron, M., Boyd, J. & Campbell, I.D. (1992) Cell 71, 671-678), Asp 7 and 23, and Glu 9 form a patch on thesurface (FIG. 21), with Asp 7 centrally located in the patch. Thus, itis reasonable to expect that these residues influence each other'sionization profile. In order to identify which of the three residueshave a highly upshifted pK_(a) the H(C)CO spectrum of the protein in 99%D₂O buffer at pH*5.0 (direct pH meter reading) was then collected. Asp23 and Glu 9 showed larger deuterium isotope shifts (0.33 and 0.32 ppm,respectively) than Asp 7 (0.18 ppm). These results show that Asp 23 andGlu 9 are protonated to a greater degree than Asp 7. Thus, we concludedthat Asp 23 and Glu 9 have highly upshifted pK_(a)'s, due to stronginfluence of Asp 7.

Mutational Analysis

The spatial proximity of Asp 7 and 23, and Glu 9 explains theunfavorable electrostatic interactions in FNfn10 identified in thisstudy. At low pH where these residues are protonated and neutral, therepulsive interactions are expected to be mostly relieved. Thus, itshould be possible to improve the stability of FNfn10 at neutral pH, byremoving the electrostatic repulsion between these three residues.Because Asp 7 is centrally located among the three residues, it wasdecided to mutate Asp 7. Two mutants, D7N and D7K were prepared. Theformer neutralizes the negative charge with a residue of virtuallyidentical size. The latter places a positive charge at residue 7 andincreases the size of the side chain.

The ¹H, ¹⁵N-HSQC spectra of the two mutant proteins were nearlyidentical to that of the wild-type protein, indicating that thesemutations did not cause large structural perturbations (data not shown).The degrees of stability of the mutant proteins were then characterizedusing thermal and chemical denaturation measurements. Thermaldenaturation measurements were performed initially with 100 mM sodiumchloride, and 6.3 M urea was included to ensure reversible denaturationand to decrease the temperature of the thermal transition. All theproteins were predominantly folded in 6.3 M urea at room temperature.All the proteins underwent a cooperative transition, and the two mutantswere found to be significantly more stable than the wild type at neutralpH (FIG. 22 and Table 10). Furthermore, these mutations almosteliminated the pH dependence of the conformational stability of FNfn10.These results confirmed that destabilizing interactions involving Asp 7in wild-type FNfn10 at neutral pH are the primary cause of the pHdependence.

TABLE 10 The midpoint of thermal denaturation (in ° C.) of wild-type andmutant FN3 in the presence of 6.3M urea. pH 2.4 pH 7.0 Protein 0.1M NaCl1M NaCl 0.1M NaCl 1M NaCl wild type 72 82 62 70 D7N 68 82 69 80 D7K 6977 70 78 The error in the midpoints for the 0.1M NaCl data is ±0.5° C.Because most of the 1M NaCl data did not have a sufficient baseline forthe denatured state, the error in the midpoints for these data wasestimated to be ±2° C.

The effect of increased sodium chloride concentration on theconformational stability of the wild type and the two mutant proteinswas next investigated. All proteins were more stable in 1 M sodiumchloride than in 0.1 M sodium chloride (FIG. 22). The increase of thesodium chloride concentration elevated the T_(m) of the mutant proteinsby approximately 10° C. at both acidic and neutral pH (Table 10).Remarkably the wild-type protein was also equally stabilized at both pH,although it contains unfavorable interactions among the carboxyl groupsat neutral pH but not at acidic pH.

Chemical denaturation of FNfn10 proteins was monitored usingfluorescence emission from the single Trp residue of FNfn10 (FIG. 23).The free energies of unfolding at pH 6.0 and 4 M GuHCl were determinedto be 1.1 (±0.3), 1.7 ((0.2) and 1.4 (±0.1) kcal/mol for the wild type,D7N and D7K, respectively, indicating that the two mutations alsoincreased the conformational stability against chemical denaturation.

Determination of the pK_(a)'s of the Side Chain Carboxyl Groups in theMutant Proteins

The ionization properties of carboxyl groups in the two mutant proteinswas investigated. The 2D H(C)CO spectra of the mutant proteins at thehigh and low ends of the pH titration (pH˜7 and ˜1.5, respectively) werenearly identical to the respective spectra of the wild type, except forthe loss of the cross peaks for Asp 7 (data not shown). This similarityallowed for an unambiguous assignment of resonances of the mutants,based on the assignments for wild-type FNfn10. The pH titrationexperiments revealed that, except for Glu 9 and Asp 23, the behaviors ofAsp and Glu carboxyl groups are very close to their counterparts in thewild-type protein (FIG. 24 Panels A, C, D, F and G, and Table 9),indicating that the two mutations have marginal effects on theelectrostatic environments for these carboxylates. In contrast, thetitration curves for E9 and D23 show significant changes upon mutation(FIG. 24 Panels B and E). The pK_(a) of D23 was lowered by more than 1.6and 1.4 pH units in the D7N and D7K mutants, respectively. These resultsclearly show that the repulsive interaction between D7 and D23contributes to the increase in pK_(a) of Asp 23 in the wild-typeprotein, and that it was eliminated by the neutralization of thenegative charge at residue 7. The pK_(a) of Glu 9 was reduced by 0.4 pHunit by the D7N mutation, while it was decreased by 0.8 pH units in theD7K mutant. The greater reduction of Glu 9 pK_(a) by the D7K mutationsuggests that there is a favorable interaction between Lys 7 and Glu 9in this mutant protein.

Discussion

The present inventor has identified unfavorable electrostaticinteractions in FNfn10, and improved its conformational stability bymutations on the protein surface. The results demonstrate that repulsiveinteractions between like charges on protein surface significantlydestabilize a protein. The results are also consistent with recentreports by other groups (Loladze, V. V., Ibarra-Molero, B.,Sanchez-Ruiz, J. M. & Makhatadze, G. I. (1999) Biochemistry 38,16419-16423; Perl, D., Mueller, U., Heinemann, U. & Schmid, F. X. (2000)Nat Struct Biol 7, 380-383; Spector, S., Wang, M., Carp, S. A., Robblee,J., Hendsch, Z. S., Fairman, R., Tidor, B. & Raleigh, D. P. (2000)Biochemistry 39, 872-879; Grimsley, G. R., Shaw, K. L., Fee, L. R.,Alston, R. W., Huyghues-Despointes, B. M., Thurlkill, R. L., Scholtz, J:M. & Pace, C. N. (1999) Protein Sci 8, 1843-1849), in which proteinstability was improved by eliminating unfavorable electrostaticinteractions on the surface. In these studies, candidates for mutationswere identified by electrostatic calculations (Loladze, V. V.,Ibarra-Molero, B., Sanchez-Ruiz, J. M. & Makhatadze, G. I. (1999)Biochemistry 38, 16419-16423; Spector, S., Wang, M., Carp, S. A.,Robblee, J., Hendsch, Z. S., Fairman, R., Tidor, B. & Raleigh, D. P.(2000) Biochemistry 39, 872-879; Grimsley, G. R., Shaw, K. L., Fee, L.R., Alston, R. W., Huyghues-Despointes, B. M., Thurlkill, R. L.,Scholtz, J. M. & Pace, C. N. (1999) Protein Sci 8, 1843-1849) or bysequence comparison of homologous proteins with different stability(Pert, D., Mueller, U., Heinemann, U. & Schmid, F. X. (2000) Nat StructBiol 7, 380-383). The present strategy using pK_(a) determination usingNMR has both advantages and disadvantages over the other strategies. Thepresent method directly identifies residues that destabilize a protein.Also it does not depend on the availability of the high-resolutionstructure of the protein of interest. Electrostatic calculations mayhave large errors due to the flexibility of amino acid side chains onthe surface, and the uncertainty in the dielectric constant on theprotein surface and in the protein interior. For example, in the NMRstructure of FNfn10 (Main, A. L., Harvey, T. S., Baron, M., Boyd, J. &Campbell, I. D. (1992) Cell 71, 671-678), the root mean squareddeviations among 16 model structures for the O^(∈) atom of Glu residuesare 1.2-2.4 Å, and those for Lys N^(ξ) atoms are 1.5-3.1 Å. Suchuncertainties in atom position can potentially cause large differencesin calculation results. On the other hand, the present strategy requiresthe NMR assignments for carboxyl residues, and NMR measurements over awide pH range. Although recent advances in NMR spectroscopy have made itstraightforward to obtain resonance assignments for a small protein,some proteins may not be sufficiently soluble over the desired pH range.In addition, knowledge of the pK_(a) values of ionizable groups in thedenatured state is necessary for accurately evaluating contributions ofindividual residues to stability (Yang, A.-S. & Honig, B. (1992) Curr.Opin. Struct. Biol. 2, 40-45). Kuhlman et al. (Kuhlman, B., Luisi, D.L., Young, P. & Raleigh, D. P. (1999) Biochemistry 38, 4896-4903) showedthat pK_(a)'s of carboxylates in the denatured state has a considerablylarge range than those obtained from small model compounds. Despitethese limitations, the present method is applicable to many proteins.

The inventor showed that the unfavorable interactions involving thecarboxyl groups of Asp 7, Glu 9 and Asp23 were no longer present ifthese groups are protonated at low pH or if Asp 7 was replaced with Asnor Lys. The similarity in the measured stability of the mutants and thewild type at low pH (Table 10) suggests that no other factorssignificantly contribute to the pH dependence of FNfn10 stability andthat the mutations caused minimal structural perturbations. The littlestructural perturbation was expected, since the carboxyl groups of thesethree residues are at least 50% exposed to the solvent, based on thesolvent accessible surface area calculation on the NMR structure (Main,A. L., Harvey, T. S., Baron, M., Boyd, J. & Campbell, I. D. (1992) Cell71, 671-678).

The difference in thermal stability of the wild-type protein betweenacidic and neutral pH persisted in 1 M sodium chloride (Table 10).Likewise, the wild-type protein exhibited a large pH-dependence instability in 4 M GuHCl (FIG. 18). Furthermore, upon the increase in thesodium chloride concentration from 0.1 to 1.0 M, the T_(m) of thewild-type and mutant proteins all increased by ˜10° C., which is in thesame magnitude as the change in T_(m) of the wild type by the pH shift.These data indicate that the unfavorable interactions identified in thisstudy were not effectively shielded in 1 M NaCl or in 4 M GuHCl. Becausethe effect of increased sodium chloride was uniform, this stabilizationeffect of sodium chloride is likely due to the nonspecific salting-outeffect (Timasheff, S, N. (1992) Curr. Op. Struct. Biol. 2, 35-39). Othergroups also reported little shielding effect of salts on electrostaticinteractions (Perutz, M. F., Gronenborn, A. M., Clore, G. M., Fogg, J.H. & Shih, D. T. (1985) J Mol Biol 183, 491-498; Hendsch, Z. S.,Jonsson, T., Sauer, R. T. & Tidor, B. (1996) Biochemistry 35,7621-7625). Electrostatic interactions are often thought to diminishwith increasing ionic strength, particularly if the site of interactionis highly exposed. Accordingly, the present data at neutral pH (Table10) showing no difference in the salt sensitivity between the wild typeand the mutants could be interpreted as Asp 7 not being responsible fordestabilizing electrostatic interactions. Although the reason for thissalt insensitivity is not yet clear, the present results provide acautionary note on concluding the presence and absence of electrostaticinteractions solely based on salt concentration dependence.

The carboxyl triad (Asp 7 and 23, and Glu 9) is highly conserved inFNfn10 from nine different organisms that were available in the proteinsequence databank at National Center for Biotechnology Information (seethe World-Wide-Web at ncbi.nlm.nih.gov). In these FNfn10 sequences, Asp9 is conserved except one case where it is replaced with Asn, and Glu 9is completely conserved. The position 23 is either Asp or Glu,preserving the negative charge. As was discovered in this study, theinteractions among these residues are destabilizing. Thus, their highconservation, despite their negative effects on stability, suggests thatthese residues have functional importance in the biology of fibronectin.In the structure of a four-FN3 segment of human fibronectin (Leahy, D.J., Aukhil, I. & Erickson, H. P. (1996) Cell 84, 155-164), theseresidues are not directly involved in interactions with adjacentdomains. Also these residues are located on the opposite face of FNfn10from the integrin-binding RGD sequence in the FG loop (FIG. 21).Therefore, it is not clear why these destabilizing residues are almostcompletely conserved in FNfn10. In contrast, no other FN3 domains inhuman fibronectin contain this carboxyl triad (for a sequence alignment,see ref Main, A. L., Harvey, T. S., Baron, M., Boyd, J. & Campbell, I.D. (1992) Cell 71, 671-678). The carboxyl triad of FNfn10 may beinvolved in important interactions that have not been identified todate.

Clarke et al. (Clarke, J., Hamill, S. J. & Johnson, C. M. (1997) J MolBiol 270, 771-778) reported that the stability of the third FN3 of humantenascin (TNfn3) increases as pH was decreased from 7 to 5. Althoughthey could not perform stability measurements below pH 5 due to proteinaggregation, the pH dependence of TNfn3 resembles that of FNfn10 shownin FIG. 18. TNfn3 does not contain the carboxylate triad at positions 7,9 and 23 (Leahy, D. J., Hendrickson, W. A., Aukhil, I. & Erickson, H. P.(1992) Science 258, 987-991), indicating that the destabilization ofTNfn3 at neutral pH is caused by a different mechanism from that forFNfn10. A visual inspection of the TNfn3 structure revealed that it hasa large number of carboxyl groups, and that Glu 834 and Asp 850(numbering according to ref Leahy, D. J., Hendrickson, W. A., Aukhil, I.& Erickson, H. P. (1992) Science 258, 987-991) forms a cross-strandpair. It will be interesting to examine whether altering this pair canincrease the stability of TNfn3.

In conclusion, a strategy has been described to experimentally identifyunfavorable electrostatic interactions on the protein surface andimprove the protein stability by relieving such interactions. Thepresent results have demonstrated that forming a repulsive interactionbetween carboxyl groups significantly destabilize a protein. This is incontrast to the small contributions of forming a solvent-exposed ionpair. Unfavorable electrostatic interactions on the surface seem quitecommon in natural proteins. Therefore, optimization of the surfaceelectrostatic properties provides a generally applicable strategy forincreasing protein stability (Loladze, V. V., Ibarra-Molero, B.,Sanchez-Ruiz, J. M. & Makhatadze, G. I. (1999) Biochemistry 38,16419-16423; Perl, D., Mueller, U., Heinemann, U. & Schmid, F. X. (2000)Nat Struct Biol 7, 380-383; Spector, S., Wang, M., Carp, S. A., Robblee,J., Hendsch, Z. S., Fairman, R., Tidor, B. & Raleigh, D. P. (2000)Biochemistry 39, 872-879; Grimsley, G. R., Shaw, K. L., Fee, L. R.,Alston, R. W., Huyghues-Despointes, B. M., Thurlkill, R. L., Scholtz, J.M. & Pace, C. N. (1999) Protein Sci 8, 1843-1849). In addition,repulsive interactions between carboxylates can be exploited fordestabilizing undesirable, alternate conformations in protein design(“negative design”).

Example XX An Extension of the Carboxyl-Terminus of the MonobodyScaffold

The wild-type protein used for stability measurements is described underExample 19. The carboxyl-terminus of the monobody scaffold was extendedby four amino acid residues, namely, amino acid residues(Glu-Ile-Asp-Lys) (SEQ ID NO:119), which are the ones that immediatelyfollow FNfn10 of human fibronectin. The extension was introduced intothe FNfn10 gene using standard PCR methods. Stability measurements wereperformed as described under Example 19. The free energy of unfolding ofthe extended protein was 7.4 kcal mol⁻¹ at pH 6.0 and 30° C., very closeto that of the wild-type protein (7.7 kcal mol⁻¹). These resultsdemonstrate that the C-terminus of the monobody scaffold can be extendedwithout decreasing its stability.

Example XXI Reconstitution of Proteins

Design and Production of Fragments by Cyanogen Bromide Cleavage of aMutant FNfn10

Choice of Cyanogen Bromide for the Cleavage

To produce fragments of the FNfn10 protein, a cleavage site wasengineered into loop regions. The insertion of a methionine residueflanked by two glycine residues on each side presented a cyanogenbromide cleavage site in a flexible region. This method had the benefitthat the protein could be expressed and purified with already existingprotocols, and both fragments were produced at the same time. Since nomethionine residue is present in the wild type sequence of FNfn10, thismethod allowed specific cleavage at the introduced site.

Location of the Introduced Cleavage Site within FNfn10

A suitable site for the separation of two fragments of FNfn10 had to bedetermined. Both practical aspects of the cleavage and its intendedapplication in selection experiments constrained the position of thecleavage site. A cleavage site within a more flexible loop region ismore likely to result in protein reconstitution. With libraryconstruction in mind, an ideal split of the protein would result infragments that each contained a portion of the molecule amendable forthe introduction of a library. In the original design, the BC and FGloop have been utilized to host restrained peptide libraries, thereforethese loops should ideally remain uncut (see FIG. 1B, 1D). The DE loopis also a potential cleavage site, though its proximity to the FG andthe BC loop may interfere target binding of BC and/or FG loops. Toseparate BC and FG loops into two different fragments, the CD loop orthe EF loop region remained as possible cleavage sites.

The elongation of loop regions introduces a destabilizing effect on theprotein conformation. In FNfn10, the destabilization effect ofmodifications in the CD loop tended to be much less pronounced than theeffect of modifications in the EF loop. The cleavage of a peptide bondin a loop region introduces an increased degree of freedom to nearbyresidues compared to loop elongation. For reconstitution, a more stableprotein should give an advantage. Consequently, it was tested whether astabilizing mutation far away from the cleavage site, such as a surfacecharge alteration that has been demonstrated (Koide et al. 2001),increase the affinity of a reconstitution reaction. To that end, a totalof four mutations for cleavage were constructed (Table 11).

TABLE 11 Constructed proteins for fragmentation  experiments and their free energy of   unfolding ΔG. ΔG₀ ΔG_(3MGuHCl)Basis (kcal/ (kcal/ m(kcal Name protein Insertion mol) mol) mol⁻¹M⁻¹)CD  Wild  GGMGA 6.4 1.4 1.69 45 type (SEQ ID NO: 142) in CD- loop CD92D7KE9Q GGMGG 7.9 1.6 2.10 (SEQ ID NO: 122) in CD- loop EF  Wild  GGMGG6.3 −1.0 2.43 45 type (SEQ ID NO: 122) in EF- loop EF92 D7KE9Q GGMGG 6.9−1.2 2.69 (SEQ ID NO: 122) in EF- loopEither the wild type or the D7KE9Q mutant were used as the template. Thecleavage site, GGMGG (SEQ ID NO:122), was inserted either in the CD orEF loop regions (see FIG. 25).Constructs to Obtain Mutant Protein

Site directed mutagenesis was performed on the FNfn10 gene to obtainexpression vectors for mutant proteins that contained a GGMGG (SEQ IDNO:122) insertion in either the CD loop or the EF loop. The insertionwas encoded in oligonucleotides (Operon Technologies Inc.), which wasused to produce the N-terminal part of the FNfn10 gene by standard PCR.The purified DNA fragment was cut at an existing NdeI site and either atan EcoRI or a SalI restriction site to process the gene for the CD orthe EF loop insertion respectively. Following the digest, the fragmentswere ligated into a suitably cut parental vector. All constructs wereconfirmed by gene sequencing. Unfortunately, one of the CD loopinsertion mutants contained a glycine to alanine mutation within theinserted glycines. Since the purpose of the glycine was solely toprovide a flexible environment around the methionine residue for moreeffective cleavage, no significant alteration was expected due to thismutation. Nevertheless, only the N-terminal fragment of this protein wasused in experiments. For the reconstitution of this protein, theC-terminal fragment of CD 92 was utilized, as it was designed to beexactly the same as for the wild type CD loop insertion (CD45),including the artificial GG sequence at the beginning instead of GA. Allthe mutant proteins were expressed as soluble protein and subsequentlypurified using metal affinity chromatography, as previously describedfor the wild type protein (Koide et al. 1998).

Residues 1-42 of FN3 were also expressed as a fusion protein, His6-ubiquitin-FN3(residue 1-42) (6×His disclosed as SEQ ID NO:136) bycloning the gene corresponding to the FN3 fragment in a vector forubiquitin (Kohno, 1998). This fusion protein was expressed and purifiedas described before (Koide et al. 1998) except that protein purificationwas performed in 4M urea.

Protein Cleavage and Fragment Purification

Protein was diluted into 0.1M HCl at protein concentrations of approx. 2mg/ml and degassed. Approximately 2-5 mg of cyanogen bromide (CNBr) wasdissolved and the reaction container was sealed under Argon to minimizetryptophan oxidation, and incubated for 2 h at room temperature. Thewell-established reaction of CNBr cleaving the peptide bond following amethionine is shown in FIG. 29. The reaction mixture was then passedthrough a single-use reverse phase cartridge (Waters) to remove anyremaining CNBr and bound proteins were recovered by eluting with 0.1MHCl containing 60% Acetonitrile (CH₃CN) and kept on ice. Elutionfractions that exhibited significant IJV absorption were combined anddiluted to approx. 25% CH₃CN. The samples were immediately loaded onto areverse phase column (Resource™ RPC, Pharmacia Amersham) and fragmentswere separated by a CH₃CN gradient from 20% to 45% (see FIG. 30). Elutedfractions containing pure fragment were immediately frozen to −80° C.and lyophilized to minimize acid degradation.

NMR Spectroscopy

NMR experiments were performed at 30° C. on an INOVA 600 spectrometer(Varian Instruments). ¹⁵N-labeled sample of the CD92 protein (see Table11) was expressed and purified. A ¹H, ¹⁵N-HSQC spectrum of the uncleavedprotein was recorded in 20 mM sodium phosphate buffer at pH 6.0containing 100 mM sodium chloride and 5% (v/v) deuterium oxide at asample concentration of 0.95 mM. as described previously (Kay et al.1992). The labeled protein sample was then recovered, cleaved withapproximately 10× molar excess of cyanogen bromide and the resultingfragments purified. Experiments on the fragments were also performed in20 mM sodium phosphate buffer at pH 6.0 containing 100 mM sodiumchloride and 5% (v/v) deuterium oxide on samples at a concentration of0.5 mM for the C-terminal fragment and 0.25 mM for the N-terminalfragment sample. 10% glycerol was added to the N-terminal fragmentsample to prevent aggregation. The two samples for the reconstitutedcomplex were prepared by dissolving both C and N terminal fragments in apH 6.0 buffer containing 4 M GuHCl, where one of the fragments in thesample was ¹⁵N-labeled while the complementary part was not. GuHCl wasthen gradually diluted out by a series of dialyses. Additionally, thesamples were concentrated and buffer exchanged using a Centricon® spinfilter (Amicon® Inc.) with a molecular weight cutoff at 3 kDa. Sampleconcentration for the complex was measured to 0.2 mM of ¹⁵N-labeledfragment respectively. For both samples, the starting unlabeledfragments was added in 3 fold excess. NMR data were processed using theNMRPipe package (Delaglio et al. 1995), and analyzed using the NMRViewsoftware (Johnson and Blevins 1994).

Protein dynamics were probed using a heteronuclear ¹H ¹⁵N steady stateNuclear Overhauser Effect (NOE) experiment (Farrow et al. 1994). An NOEis observed due to cross relaxation of two spins that are in closeproximity (Cavanagh et al. 1996). The NOE enhancement of a coupledproton-nitrogen pair was measured as a ratio of peak volume while NOEtransfer was allowed to the peak volume of a control experiment withoutthe saturation of ¹H resonances (Kay et al. 1989).

Monitoring Reconstitution of the Fragments by Fluorescence Spectroscopy

To measure the amity of the interaction, a technique is preferable thatrequires lower sample concentration than NMR. Therefore, the reactionwas investigated using the inherent fluorescence of the tryptophanresidue present in the N-terminal domain.

Proteins were dissolved to a final concentration of 500 nM in 20 mMsodium phosphate buffer at pH 6.0 containing 100 mM sodium chloride, 750mM glycerol unless otherwise noted and various urea concentrationbetween 1 M and 2.5 M. Urea concentration was determined using an Abberefractometer (Spectronic Instruments) as described (Pace and Sholtz1997). To obtain (lath on the reconstitution at conditions without theaddition of urea and glycerol, a series of C-terminal fragment-titrationexperiments with varying urea or glycerol concentrations, respectively,were performed. The dissociation constants in the absence of urea orglycerol were estimated by extrapolation based on experimental data.

The reconstitution reaction follows the scheme

where: (1)

$K_{D} = \frac{\lbrack N\rbrack*\lbrack C\rbrack}{\lbrack {NC}_{complex} \rbrack}$With: (2,3)[N]+[NC _(complex) ]=[N] ₀[C]+[NC _(complex) ]=[C] ₀the equation results in: (4)

$\lbrack N\rbrack = {\frac{( {\lbrack N\rbrack_{0} - \lbrack C\rbrack_{0} - K_{D}} )}{2} + {\frac{1}{2}\sqrt{( {\lbrack N\rbrack_{0} - \lbrack C\rbrack_{0} - K_{D}} )^{2} + {4*K_{D}*\lbrack N\rbrack_{0}}}}}$where [N] is the concentration of free N-terminal fragment,[NC_(complex)] the concentration of complex in solution, [N]₀ the totalconcentration of N-terminal fragment, which is the fluorophore. Theconcentration of the C-terminal fragment [C] and [C]₀ is treatedsimilarly. This relationship allows the fitting of titration experimentswhere the fluorescence F is fitted by: (5)

$F = {{\frac{\lbrack N\rbrack}{\lbrack N\rbrack_{0}}*F_{isolated}} + {( {1 - \frac{\lbrack N\rbrack}{\lbrack N\rbrack_{0}}} )*F_{complex}}}$F_(isolated) and F_(complex) represent two more fitting parameters givenby the starting and the asymptotic endpoint of a titration. Theresulting fluorescence decay at a particular wavelength was fitted foreach titration experiment, which required the use of four fittingparameters: [N-terminal], F_(isolated) and F_(complex), and thedissociation constant of the reconstitution. Even though approximatevalues of the first three parameters were known, all parameters wereallowed to vary in order to obtain a best fit. The fitting resulted inparameters close to the expected, approximate values and restrictingparameters to the expectation values lead only to minor changes in theobserved dissociation constant.

The linear dependency of the free energy of unfolding of many proteinson urea concentration has been well established. As reconstitutionresults in a folding of the peptides comparable to the folding of theintact protein, the free energy of the reconstitution was assumed todepend linearly on urea concentration as well. As the free energy of thereconstitution depends exponentially on the dissociation constant, thedependence of the K_(D) on the urea concentration required a linear fitof the logarithm of the K_(D) (see FIG. 39). Glycerol concentration waskept constant at 750 mM in this set of experiments. For the glycerolconcentration dependence, no precedent has been established in theliterature. The dissociation constant was therefore assumed to dependlinearly on glycerol concentration (see FIG. 40). A logarithmic fitsimilar to the urea concentration dependence did not represent the datawell. However, the dependence of the dissociation constant on glycerolwas not very strong, and thus similar values were obtained even if otherdependencies were assumed.

Circular Dichroism

The k UV-circular dichroism spectrum of C-terminal fragment was recordedat concentrations of 5 μM to 100 μM in 20 mM sodium phosphate buffer (pH6.0) containing 100 mM sodium chloride. A sample at 1.5 μM fragmentconcentration was investigated to test for the presence of secondarystructure in buffer conditions of the fluorescence experiments (sodiumphosphate buffer with 1M urea and 750 mM glycerol). The N-terminalfragment was measured at a fragment concentration of 5 μM in 20 mMsodium phosphate buffer (pH 6.0) containing 100 mM sodium chloride and750 mM glycerol. Circular dichroism measurements were performed using aModel 202 spectrometer equipped with a Peltier temperature controller(Aviv Instruments) using a 1 cm pathlength.

The temperature dependence of the secondary structure in the C-terminalfragment was investigated as well. The maximum of the inflection in thespectrum at low temperature at 232 nm (see FIG. 41) was recorded as thesample temperature was raised at a rate of approximately 1° C. perminute. The thermal denaturation data were fitted with the standardtwo-state model (Pace and Sholtz 1997):ΔG(T)=ΔH _(m)(1−T/T _(m))−ΔC _(p)[(T _(m) −T)+T ln(T/T _(m))]where ΔG(T) is the Gibbs free energy of unfolding at temperature T,ΔH_(m) is the enthalpy change upon unfolding at the midpoint of thetransition, T_(m), and ΔC_(p) is the heat capacity change uponunfolding. A ΔC_(p) was approximated to 1.04 kcal mol⁻¹ K⁻¹, based onMyers et al. (Myers et al. 1995) and kept constant for all measurements.ResultsDenaturant Induced Unfolding of Proteins with Inserted Cleavage Site

The stability of the mutant proteins was investigated before cleavageusing guanidine hydrochloride induced unfolding and refolding reactionsmonitored by tryptophan fluorescence. The fluorescence was quenched inthe folded protein and allowed a convenient way to measure the unfoldingtransition. The unfolding curves of all four proteins are shown in FIGS.26A and 26B. The standard two state transition was assumed in theanalysis and resulting parameters of the fitting are given in Table 11.The difference in stability between CD loop and EF loop elongationmutant was observed for these mutants featuring a five-residueinsertion. The effect of the altered surface charge D7KE9Q at theN-terminal end was difficult to judge from this measurement, since theunfolding reaction in GuHCl was shown least sensitive to the mutationwith only a marginal increase in stability (see FIG. 26B). For theunfolding of the proteins with a cleavage site, no stabilizing effect ofthe D7KE9Q mutations within error was seen. The additional methioninehad no unexpected effect in addition to what was seen in the fourglycine insertion.

Separation of N- and C-Terminal Fragments

The peptide bond following a methionine residue was cleaved under acidicconditions using cyanogen bromide. Initial tests exhibited successful,albeit incomplete cleavage of all the proteins at mildly acidicconditions. Further optimization for the preparation was found at moreacidic reaction and purification conditions and a strict limitation ofcleavage time to minimize deamidation under the acidic conditions. Atypical reverse phase chromatogram for the CD loop cleavage is shown inFIG. 28. The N- and C-terminal fragments identity was confirmed by massspectroscopic analysis. Although there is no methionine in the wild typeFNfn10 sequence, there is a secondary cleavage site at the start of theprotein, separating a multiple histidine (HisTag) leader sequence fromthe N-terminal fragment. The N-terminal fragment that has the HisTagstill attached was shown to run as the contaminant peak number 2.Contaminant peak number 3 was shown to be of slightly smaller mass thanthe N-terminal fragment. Its volume increased with prolonged exposure toan acidic environment regardless of CNBr presence, and likely resultedfrom a deamidation event. Contaminant peak number 1 appeared to includeuncleaved FNfn10 as well as both fragments. Its volume was sensitive tothe exact loading conditions, where moderate amount of acetonitrilepresent in the loading buffer decreased the peak volume. Most likely,reconstitution of the fragments was taking place even at acidicconditions and complexed fragments resulted in peak number 1.

Initial Tests Revealed Different Fragment Characteristics Between CutSites

Trials to observe reconstitution using fluorescence were obstructed bythe presence of nonspecific adhesion, however, qualitative data could beobtained. The Trp fluorescence of FNfn10 was highly quenched in thefolded state. If the reconstituted complex formed the samethree-dimensional structure, quenching of the fluorescence signal wasexpected upon a reconstitution reaction. Both N-terminal fragmentsproduced by a cleavage in the CD loop, with or without the surfacecharge mutations, revealed such quenching upon the addition of theC-terminal fragment, indicating that reconstitution had occurred. Thefragments produced by a cleavage in the EF loop, however, showed noindication of reconstitution. The wild-type N-terminal fragment fromCD45 (see Table 11) exhibited a poorer solubility compared to the D7KE9Qcounterpart, CD92. The mutant CD-loop fragments were chosen for adetailed study of the reconstitution reaction.

Size Exclusion Chromatography Confirmed Reconstitution of CD CutFragments

Size exclusion chromatography was performed on the fragments of CD92using a Sephadex75 gel filtration column. When a mixture of N- andC-terminal fragment at 5 μM concentration were loaded onto the gelfiltration column (see FIG. 29) the elution showed only a single peak atthe retention time of uncut protein, compared to a significantly slowerelution of C-terminal fragment alone. The qualitative pattern isconsistent with the formation of a complex with a dissociation constantwell below 1 μM. However, both fragments in isolation appeared to bindto the column, hampering a quantitative analysis.

Fragments Reconstitute the Native Fold of the Uncut Protein

Further experiments were performed to distinguish any specific bindingfrom nonspecific binding. To investigate if the apparent complexformation was observed by gel filtration chromatography stemmed from aspecific binding event, nuclear magnetic resonance (NMR) spectroscopywas applied. NMR offers atom-specific information because nuclear spinsof an atom within a protein represent extremely sensitive probes fortheir local electromagnetic environment. The exact conformation of aprotein determines the chemical shift of a nuclear spin.

Multidimensional heteronuclear NMR spectroscopy allows the correlationof chemical shift of nearby atoms, giving rise to a distinct patternthat is specific for a conformation of a protein. It is apparent whetherthe protein is unfolded or folded since the electromagnetic environmentchanges drastically during folding for most atoms in a protein.Similarly, if two proteins form a complex due to specific interactions,the resulting pattern of chemical shifts of each nuclei will reflectchanges compared to that in isolation for residues involved in thosespecific interactions. If a fragments is disordered in isolation,chemical shifts of all atoms are clustered in a narrow regimecharacteristic for random coil, and most atoms will experiencesignificant change in their local environment and thus in chemical shiftupon folding into a structure. To test the conformation of the fragmentsin isolation and when combined, ¹⁵N labeled protein was purified and NMRanalysis was performed.

The ¹H-¹⁵N HSQC spectrum of the uncut protein exhibited a peakdistribution matching that of wild type FNfn10 for most amides (see FIG.30). As expected, there were additional peaks that likely originatedfrom the inserted residues, and there were changes in the peak positionfor residues in the immediate vicinity of the surface charge mutationsD7KE9Q. Nevertheless, the changes in chemical shift were limited tostructurally adjacent residues, e.g., D23, which were likely to beaffected. Thus, the β-sandwich fold of FNfn10 was maintained in theprotein featuring both the D7KE9Q and the cleavage site insertionmutation.

The spectrum of each fragment by itself showed limited peak dispersionindicative of an unstructured peptide (see FIG. 31). For the C-terminalfragment, additional peaks appeared that stem from a reversibletransition to an oligomeric state. When reducing the temperature to 5°C., the population of this alternative conformation was reduced,resulting in a spectrum indicative of an unfolded peptide.

The N-terminal fragment in isolation was not soluble at high proteinconcentrations and required the presence of 10% glycerol to record aspectrum (see FIG. 31). Though peak broadening indicates formation oflarger aggregates, the spectrum did not exhibit a significant spread ofchemical shifts, suggesting that the aggregate conformers wereunstructured.

Once N- and C-terminal fragments were combined, the tendency of theN-terminal fragment to aggregate decreased significantly, allowinghigher concentrations of fragment. This indicated that a more foldedcomplex was formed. An HSQC-spectrum on a complex formed of labeledN-terminal fragment and unlabeled C-terminal fragment exhibited adrastic change to a well-dispersed distribution of peaks (see FIG. 35A,B).

Similarly, addition of unlabeled N-terminal to labeled C-terminal samplerevealed a conformational change to a well-dispersed spread of chemicalshifts. The overlap of these two spectra, equivalent to the spectrum ofa fully labeled complex, was virtually identical to the previouslyrecorded spectrum of the uncut protein (see FIG. 36A; B). Therefore, thereconstitution of the two fragments resulted in the formation of acomplex that had the same fold as the original protein.

Reconstituted Protein Appears as Rigid as the Uncut Protein

The next question investigated was whether the formation of a complexwith similar fold as the original protein would result in a more dynamicassembly. The association could lead to a more loose assembly, whereregions could exhibit motion on much larger scale than possible in theuncleaved protein. Steady-state {¹H}-{¹⁵N}-NOE measurements yieldinformation on fast, picosecond to nanosecond time scale dynamics of amolecule. For a qualitative judgement of the overall changes indynamics, a full assignment of the resonances is not necessary, as it isnot important to identify particular residues at this point. Thus, theNOE experiment was analyzed for each peak found in the investigatedspectra (see FIGS. 35 and 36). A tentative assignment based on thesimilarity of the complex spectrum to the known assignment of wild typeFNfn10 was shown as well.

For both N-terminal and C-terminal fragment in the complex the ¹⁵N-NOEsignal was predominantly above 0.75, indicating a rigid assemblycomparable to the uncut protein in this motion regime. In contrast, theisolated C-terminal fragment showed significantly lower values for themajority of resonances, characteristic of a flexible peptide in a randomcoil conformation. The lack of fast dynamic motion further showed thatthe reconstitution resulted in a fragment complex that had very similarcharacteristics comparable to the uncleaved protein.

Determination of the Dissociation Constant of the ReconstitutionReaction

Glycerol and Urea Limit Non-Specific Binding

Initial tests had already confirmed that the reconstituted complexexhibited similar quenching of the signal as the uncut protein, whichwas to be expected after NMR experiments had confirmed that the samethree-dimensional structure was formed. When the N-terminal fragmentcontaining the fluorophore was studied in isolation, the observedfluorescence appeared inconsistent. Further investigation revealed thatthe fluorescence of the N-terminal fragment decreased over time when thesample was kept in the quartz cuvette for the measurement (see FIG. 37).This was due to adherence of the fragment to the cuvette walls. Theeffect was also observed on plastic surfaces of storage tubes. It wasmost prevalent at the low concentration used in the fluorescenceexperiments. The adherence was found to be on a slower time scale, notcoming to an equilibrium within minutes and also was found to bereversible on a slow time scale. The exponential decay of fluorescencesignal interfered with the detection of reconstitution as quenching ofthe reconstitution and signal loss were indistinguishable. A series ofdifferent sample buffer conditions were tested. It was found that theaddition of glycerol and denaturing co-solutes such as urea or guanidinehydrochloride decreased the magnitude of adherence. At concentration of750 mM glycerol and 1M urea or higher, the fluorescence signal was foundto be nearly constant over time.

Unusually High Affinity of the Reconstitution

A dissociation constant can be determined from a titration experimentwhere the concentration of the fluorophore is held constant and itscorresponding binding partner is added. At a glycerol concentration of750 mM, 1M urea and the N-terminal fragment of approximately 500 nM, thetitration of the C-terminal fragment was fitted to approximately 10 nM(see FIG. 38). However a K_(D) of 0.1 nM resulted in a nearly identicalfit, indicating that the dissociation constant lies outside theaccurately assessable range. When measuring the dissociation constant,the concentration of the fluorophore has to be lower than or near thedissociation constant. To obtain a more accurate value, a series oftitration experiments were performed at higher urea concentration,followed by subsequent extrapolation to compensate for the addition ofurea. The resulting dissociation constants over the concentration ofUrea present are shown in FIG. 39. The line in FIG. 39 indicates thatthe detection limit of this method was set at 10 nM, which is 50× lowerthan the concentration of fluorophore. Accurate determination of thedissociation constant was no longer possible at or below this limit. Asindicated by the linear fit, an extrapolation to the absence of urea wasmade with reasonable accuracy. The dissociation constant in the absenceof urea, but in the presence of 750 mM glycerol, was estimated to be 1.5nM.

Interestingly; the reconstitution reaction equilibrated in 20-30seconds, much slower than expected for the measured high affinity.Partially, a delay was necessary for the diffusion of a small titratedaliquot throughout the sample containing glycerol and urea. In addition,a competing reaction could be responsible, for example the dissociationof an oligomeric state.

An additional series of experiments was performed to extrapolate overglycerol concentration as well. Here, no relationship had previouslybeen established. Thus, simple models were applied to obtain a fit thatyielded best match with the experimental data. The data indicate thatthe results were best represented by a direct linear correlation toglycerol concentration, as displayed in FIG. 40.

To gain an extrapolation that excluded both glycerol and urea, astepwise approach was taken that assumed that the two co-solutes hadindependent effects on the dissociation constant. Here a firstextrapolation resulted in an adjusted value in the absence of urea. Asecond extrapolation was then adjusted to exclude the presence ofglycerol, resulting in a dissociation constant of 3.6 nM in plainbuffer. An alternative path by reversing order of the extrapolationyielded a value of 3.9 nM, which was in reasonable agreement with theprevious one.

The dissociatiation constant for the C-terminal fragment and theHis6-ubiquitin-N-terminal fragment (fusion protein) was also determinedusing the fluorescence method described above. Unlike the freeN-terminal fragment, the ubiquitin fusion protein remained soluble at0.5 nM in the absence of urea or glycerol at pH 2.4 (20 mM glycine HCLbuffer containing 100 mM sodium chloride). In these conditions, thedissociation constants was determined to be 14.4±0.2 nM. This is closeto the value determined for the fragments generated from the chemicalcleavage of FN3 described above. These results indicate that connectinga foreign protein at the N-terminus of FN3 does not inhibit the fragmentreconstitution reaction.

Indication of an Oligomeric Structure in the C-Terminal Fragment

In addition to the observation of secondary signals at highertemperature in the NMR experiments, the circular dichroism spectrum ofthe C-terminal fragment showed evidence of secondary structure. Thespectrum showed an inflection in the far UV regime at 230-235 nm (seeFIG. 41), which has been associated with β-hairpin structures.

As shown in FIG. 42, B-turn structures exhibited a cooperativetemperature dependence, with a melting curve of around 37° C. for aprotein concentration of 50 μM. The phenomenon was concentrationdependent, which was a clear indication that it involved anoligomerization process, not an intramolecular folding reaction. Boththe midpoint of the melting curve as well as the cooperativity of thereaction changed when the concentration was varied. At a concentration1.5 μM C-terminal fragment in the presence of 1M urea and 750 mMglycerol the temperature increase resulted in a dependency that couldonly be fitted if the same baseline slope seen at higher concentrationwas assumed (see FIG. 42). The higher baseline matched that of thehigher concentration measurements, while the lower one was outside thetemperature range, given the low cooperativity of the transition.Nevertheless, it was possible to distinguish a transition even at thislow concentration.

Discussion

The reconstitution of the FNfn10 fragments generated from a cleavage inthe CD loop could be observed and the formation of the originalstructure was demonstrated utilizing fluorescence, gel filtration andNMR experiments. The NMR spectra indicated that the structure of theFNfn10 domain was reestablished. The NMR data also demonstrate that thewhole complex was as rigid as the uncut protein.

The dissociation constant of the reconstitution was determined to be 3.6nM using fluorescence spectroscopy. Fragments of a number of proteinshave been reported to reconstitute structure and function. However, onlya few reports the dissociation constants of the reconstitution reaction.The K_(d) of the reconstitution of the CD92 protein is one of the lowestreported values to date (Table 12).

TABLE 12 Comparison of K_(D) values for reconstitution reaction reportedin the literature. Number of Protein K_(D) (nM) Residues CommentReference FNfn10 3.6 42 + 47 (our data) Chymotrypsin 40 40 + 24(Ladurner Inhibitor-2 et al. 1997) (wild type) Ubiquitin 38000 35 + 40(Jourdan and Searle 2000) Protein G 10000 40 + 15 (Honda et al. B1domain 1999) Barnase 600 36 + 73 (Sancho and Fersht 1992) S-protein/ 599 20 + 104 Large unit (Dwyer et al. S-peptide (wild type) folds 2001)independent S-protein/ 5.4  20 + 104 Improvement (Dwyer LB2 variant(best selected) by phase et al. 2001) S-peptide display Calbindin 0.00343 + 31 Ca²⁺ (Berggard D9K dependent, et al. 2001) EF-hands Fragmentsfold independ- entlyOnly one case (Berggard et al. 2001) reported a lower value, though inthat particular case, both fragments folded independently and the Ca²⁺binding was essential for the reconstitution. Metal binding has beenknown to stabilize the three dimensional structure of proteins[Savchenko, 2002 #1248] (Lee et al. 1989) [Pabo, 2001 #1186][Li, 2001#1251], which likely applies to a reconstitution as well. This mightimpede direct comparison to this reconstitution reaction with theothers. The high affinity of the FNfn10 fragments compared to otherreconstitution reactions is consistent with a correlation to a highstability of the parental protein.

Fragment reconstitution has been reported for a number of proteinsindicating that the phenomenon is not an extraordinary characteristic(de Prat Gay and Fersht 1994; Kippen et al. 1994; Tasayco and Chao 1995;Ladurner et al. 1997; Pelletier et al. 1998; Tasayco et al. 2000;Berggard et at 2001). A similar reaction could potentially be found forany protein because the driving force to form the particularthree-dimensional structure is generally independent of the maintenanceof a single peptide bond. Cyclic permutations of proteins confirm thatif two amino acids are in proximity to each other within a fold, theaddition of a bond is generally possible as well (Zhang, 1993)(Hennecke, 1999), as long as important folding elements stay intact.However, not every peptide bond is expendable, and removing more thanone at a time may not be possible. Each peptide bond must carry someinformation on the three-dimensional. Obviously, the importance of theinformation contained in any one peptide bond varies within a protein,which gives rise to differences seen in the capability of fragments toreconstitute a protein between two cleavage sites.

Folding of a protein, and therefore reconstitution of a protein fromfragments, is primarily driven by the burial of hydrophobic surface awayfrom water. If the total surface burial upon folding from disorderedpeptides were responsible, two cleavage sites would not result indifferent affinities as the same protein is folded. If the interactionbetween fragments were needed to maintain a complex, then the burial inthe interface between the fragments is more relevant. As anapproximation, the amount of newly exposed surface in the interface uponseparation of the fragments was calculated using the Connolly algorithm(Connolly, 1983) in the program GRASP (Nicholls et al. 1991). 1930 Å²were exposed upon cleavage in the CD loop, while 1181 Å² were exposedupon cleavage in the EF loop based on the crystal structure of FNfn10(Dickenson et al. 1994). The surface area found for both cleavage siteswere comparable to binding interfaces with reasonable affinity. If theburied interface was the only determining factor, both cut sites wouldproduce fragments that reconstitute readily, and less of a difference inaffinity would be expected between the differently cut fragments.

Most cleavage sites reported were in a flexible region of thereconstituted protein. A peptide bond necessitated the proximity of tworesidues, which thereby applied a conformational constraint on thepolypeptide that was absent in a complex of fragments missing thispeptide bond. The region surrounding the cleavage site of areconstituted complex therefore exhibits an increased flexibilitycompared to the uncut protein. An entropic penalty applies to a proteinif a region is mutated to be more flexible. A region that is alreadyflexible suffers a smaller penalty upon cleavage. The highest possiblestability for a complex is achieved if a protein is cleaved in aflexible region.

The significant decrease in stability in the EF loop elongation mutantindicated the importance of interactions in the EF loop for the proteinfold. The inability of the EF loop fragments to reconstitute compared tothe CD loop fragments was correlated with the significantly lowerstability of the EF loop elongation mutant. This indicated that for amoderate to high affinity, the determining factor was the stability ofthe parental protein. As the cleaved proteins had a significantelongation inserted at the cleavage site, the stability for theseproteins had already suffered an entropic penalty. The EF loop hadsuffered an aggravated penalty due to the disruption of importantinteractions in the FNfn10 fold. The data suggest that the stability ofan insertion mutant can be utilized to predict if reconstitution ispossible.

The presence of distinct resonances in the HSQC, whose chemical shiftswere far from random coil values, was evidence for an oligomerization ofthe C-terminal fragment. Additionally, the oligomers cause aconcentration dependent inflection in the CD spectra indicative ofsecondary structure, which was not completely vanished at 1.5 μMC-terminal fragment concentration. The oligomeric structure monitored byCD exhibits a clear cooperative temperature melting (see FIG. 42). Thepresence of this inflection at the low concentration indicated thepresence of oligomers at the concentration used in the fluorescenceexperiments. However, the detected presence of distinct peaks in theHSQC and the solubility to more than 1 mM concentration suggested theabsence of large insoluble aggregates. More likely much smalleroligomers containing a limited number of molecules existed under theseconditions. Formation of larger oligomers resulted in increasedlinewidth in the NMR, similar to what was seen for the N-terminalfragment. Oligomerization of the N-terminal domain caused significantline broadening compared to the similarly sized C-terminal domain,indicating much larger oligomers. However, the N-terminal fragment didnot exhibit a more dispersed spectrum at the same time. Thus, thecomparably large oligomers were not in a distinct structure. TheC-terminal fragment exhibited characteristics in the CD and the NMRdata, indicating formation of small oligomers with a distinct structure,most likely a β-sheet conformation, that were present even at lowmicromolar concentrations.

A consequence of an oligomer formation was its competition with thereconstitution reaction. One possibility to influence the reconstitutionarises if the dissociation of an oligomer is rate-limiting for theformation of the reconstituted complex of—and C-terminal fragments. Thereconstitution complex forms rapidly, judging from the exceptionallyhigh affinity measured. The reaction is slowed by a possibledissociation of oligomer that has occur prior to reconstitution.Consistent with such a competing oligomerization reaction areobservations made in the fluorescence experiments, that equilibration ofthe complex formation is slower than expected for the measured highaffinity. Additional indication was obtained while mixing highlyconcentrated samples for the NMR experiments. Even longer equilibrationtimes and consequently careful sample preparation were necessary toattain reconstituted complex at high fragment concentration, possiblyreflecting an increased population of C-terminal fragment in theoligomeric state unavailable for the reconstitution reaction. Thedissociation of the oligomeric structure of the C-terminal domain istherefore likely to be rate limiting for the formation of thereconstituted complex.

The Reconstitution Reaction can be Observed In Vivo

Evidence on the possibilities to apply the observed reconstitution ofFNfn10 to a yeast two hybrid selection suggested that the reactionoccurs in yeast. A yeast two-hybrid selection was previously applied toisolate FNfn10 based proteins that bind to human estrogen receptorligand binding domain [Koide, 2002 #1160]. Selected proteins, termed‘monobodies’ were isolated in a ligand specific manner. Utilizingbinding proteins from this selection, yeast two-hybrid assays wereconstructed to test if monobody fragments reconstitute into a FNfn10fold in vivo. Fragments that featured either a wild type or a mutated FGloop were assayed for the occurrence of the reconstitution of FNfn10proteins (see FIG. 44). All FNfn10 reconstituted specifically in vivo.The results confirmed that all fragments with a mutant FG loopreconstituted nearly as well as the wild type fragments, indicating thatthe FG loop does not contribute significantly to FNfn10 stability.

Example XXII Reconstitution of Monobodies in Yeast Cells

This example demonstrates that the fragment reconstitution reaction ofFN3 has sufficiently high affinity and specificity as means toheterodimerize proteins of interest. The results in this example alsostrongly suggest that yeast two-hybrid libraries based on FN3 fragmentreconstitution can be constructed for large scale screening.

Strains and Media

Yeast strains EGY48, MATα his3 trp1 ura3 leu2::6 LexAop-LEU2, andRFY206, MATα his3Δ200 leu2-3 lys2Δ201 trp1Δ::hisG ura3-52, have beendescribed (Gyuris et al. 1993; Finley and Brent 1994) and were purchasedfrom Origene. Yeast was grown in YPD media or YC dropout media followinginstructions from Origene and Invitrogen. Manipulation of Escherichiacoli was according to Sambrook et al (Sambrook et al. 1989).

Constructions of Plasmids for the Yeast Two-Hybrid Screening andMonobody Libraries

The plasmids for monobody-reconstitution were constructed as follows.The plasmid pFNB42, that encodes FLAG tag-FNfn10-NLS (nuclearlocalization signal)-B42 fusion protein, was constructed by PCR. Theoligonucleotides used for the construction of the plasmids formonobody-reconstitution are found in Table 13.

TABLE 13 The oligonucleotides used for the constructionof the plasmids for monobody reconstitution Name DNA sequenceFNABCGGKpnR ACCACCGGTACCACCACCGTTACCACCGGTTT CACC (SEQ ID NO: 125)FNDGGBamF CGGGGATCCAAGGTGGTGGCTCCCCGTTCAGG AATTC (SEQ ID NO: 126)NcoFLAGFNF CATGCCATGGACTACAAGGACGACGATGACAA GGGTATGCAGGTTTCTGATGTTC(SEQ ID NO: 127) KpnGGTGGSNLSF GTGGTACCGGTGGTTCCCCTCCAAAAAAGAAG AGAAG(SEQ ID NO: 128) FNKpnGGTGGSR GGAACCACCGGTACCACCGGTACGGTAGTTAA TCGAG(SEQ ID NO: 129) B42TAAXhoR CCGACTCGAGTTAATCTCCACTCAGCAAGAG(SEQ ID NO: 131) T7F TAATACGACTCACTATAGGG (SEQ ID NO: 130) FN5RCGGGATCCTCGAGTTACTAGGTACGGTAGTTA ATCGA (SEQ ID NO: 132)The oligonucleotides NcoFLAGFNF and KpnGGTGGSR were used to amplifyFNfn10 gene from pAS45 (Koide et al. 1998), and the oligonucleotidesKpnGGTGGSNLSF and B42TAAXhoR were used to amplify NLS-B42 gene frompYesTrp2 (Invitrogen™). The two PCR fragments were annealed and extendedusing PCR, then digested with NcoI and XhoI. The fragment was ligated inpYesTrp2 that was digested with the same restriction enzymes. The FNfn10gene of pFNB42 was replaced with the gene of the N-terminal fragment ofFNfn10 (the ABC-strands of FNfn10) to construct the plasmid pFNABCB42,that encodes FLAG tag-N terminal fragment of FNfn10 (ABC strands)-NLS(nuclear localization signal)-B42 fusion protein, using the restrictionenzyme NcoI and KpnI. The gene of FNfn10 ABC strands was amplified frompFNB42 or vectors of ERαEF-binding monobodies whose AB-loop had beenmutated, using oligonucleotides T7P and FNABCGGKpnR. For theconstruction of pEGFNDEFG, that encodes LexA-C terminal fragment ofFNfn10 (DEFG strands) fusion protein, was constructed by cloning a geneof FNfn10 DEFG strands in pEG202 (Origene) using restriction enzymesBamHI and XhoI. The gene of FNfn10 DEFG strands was amplified from pAS45or the vector that encodes yeast ORF-binding monobody whose FG-loop hasbeen mutated, using oligonucleotides FNDGGBarnF and FN5R.β-Galactosidase Assay for Monobody-Reconstitution

The yeast strain EGY48 was transformed with a derivative of thepFNABCB42 plasmid encoding a fusion of N terminal fragment of particularmonobody-NLS-B42. The yeast strain RFY206 that has the plasmid pSH18-34was transformed with a derivative of the pEGFNDEFG plasmid encoding afusion of LexA-C terminal fragment of particular monobody. The EGY48strains and the RFY206 strains were mated, replicated onto YC GalRaf-his-ura-trp plate, then the β-galactosidase activity of the matedstrains was measured by agarose overlay method (Duttweiler 1996).

Results

EGY48 strains harboring a derivative of pFNABCB42, that encodes the Nterminal fragment of FNfn10-NLS-B42 fusion protein, were mated withRFY206 strains harboring β-galactosidase reporter plasmid and aderivative of pEGFNDEFG, that encodes LexA-C terminal fragment ofmonobody fusion protein. The mated strains were tested forβ-galactosidase activity, and the results are shown in FIG. 27. Theamino acid sequence of the FG loop region of the C terminal half ofmonobodies are listed on Table 14. The results show that not onlyFNfn10, but also monobodies can be reconstituted in vivo.

TABLE 14 The sequence of the FG-loop regions ofthe C-terminal half of monobodies amino acid sequence clone nameof the FG loop pEGFNDEFG VTGRGDSPASSKP (SEQ ID NO: 133) pEGFNDEFG0319VTGQWALYLSSKP (SEQ ID NO: 134) pEGFNDEFG4699 VTGGEVRCVRDAASWSSWLKP(SEQ ID NO: 135)

Example XXIII Examples of Mutations to be Introduced to Alter theAssociation Specificity of N-FN3 and C-FN3

The inventor previously demonstrated that charged residues on thesurface of FN3 have large effects on the stability of FN3 (Koide et al.,2001). Mutations of residues on the protein surface cause smallperturbations on the overall structure of a protein. Also interactionsbetween residues at “cross strand” positions (i.e., residues onneighboring beta-strands that are directly adjacent to each other) areknown to influence the beta-sheet stability (Smith & Regan, 1995).Control of peptide association using charged surface residues has beenwell documented, particularly for coiled coil peptides (see Oakley andKim, and references therein). Therefore, such mutations are used tomodulate the affinity between N-FN3 and C-FN3. Below is a generalstrategy for using N-FN3 and C-FN3 that are separated in the CD loop.Note, however, this strategy is applicable to FN3 fragments that areseparated at other points beside in the CD loop.

Strands B and E are aligned in the anti-parallel manner in one sheet ofFN3 (see FIG. 45). These two strands belong to different fragments.Mutations such as D21 (or E21) and D56 (or E56) cause electrostaticrepulsion between negative charges on strand B and strand E, thusdestabilizing the complex of N-FN3 and C-FN3. Similarly, R21 (or K21)and R56 (or K56) cause repulsion between positive charges on strands Band E, thus destabilizing the complex. In contrast, when N-FN3 with D21(or E21) and C-FN3 with R56 (or K56) are combined, the electrostaticrepulsion is eliminated, and the two fragments form a stable complex.Likewise, a combination of R21 (or K21) and E56 (or E56) also facilitatethe association. These “cross strand” positions that introduce suchmutations include residues 19 and 58, 17 and 60, and 23 and 54 onstrands B and E; residues 37 and 45, 35 and 47, 33 and 49 and 31 and 51on strands C and D; residues 37 and 69, 35 and 71, 33 and 73, and 31 and75 on strands C and F Mutations at these positions are combined toadjust the affinity and specificity of association.

For a combination of N-FN3 and C-FN3 with a different separation point,cross strand pairs are identified using the same principle.

The second class of mutations that can be used to alter the specificityof FN3 fragment reconstitution is those in the core of FN3. The core ofa protein is generally tightly packed and mutations in the core areoften highly destabilizing (Matthews 1993). Thus, multiple mutations canbe introduced in the core (for example, positions 10, 20, 36, 70 and 90can be simultaneously mutated). Because a large number of residues arein close contact in the core, one should need to introduce multiplemutations to achieve tight interaction of fragments. Core mutations andsurface mutations can be used in combination, which should provide ahigh degree of interaction specificity.

Example XXIV Procedures for Library Construction and Screening Using“Split FN3”

Nomenclature

N-FN3 and C-FN3 denote N-terminal and C-terminal fragments of FN3 thatare produced by separating FN3 at a position within a loop. For example,if the separation is within the CD loop, N-FN3 contains the A, B and Cstrands, the AB and BC loops and a section of the CD loop. C-FN3 thencontains the remaining section of the CD loop, the DE, EF, and FG loopsand the D, E, F, and G strands.

A binding pair denotes a pair of molecules that associate with eachother, having a dissociation constant of less than 10⁻⁵M⁻¹. Bindingpairs can be used to augment the association (reconstitution) of N-FN3and C-FN3. One example of a binding pair is coiled coils.

1. Phage Display

a. Two Vector System

N-FN3 is fused to a phage coat protein in such a way that it isdisplayed on the surface of bacteriophage (Kay et al., 1996; Koide etal., 1998). Alternatively, C-FN3 may be fused to a phage coat proteinsuch as pIII and pVIII. An N-terminal secretion sequence is added to thecomplementary fragment (the fragment that is not fused to a phage coatprotein) in such a way that the fragment is secreted into theperiplasmic space of Escherichia coli, Genes of these fusion proteinsare encoded on a phagemid vector, such as pBlueScript (Stratagene) underthe control of a regulatable promoter. Alternatively, the phage genomecan be used. The phagemid encoding N-FN3 contains a drug resistancemarker (such as ampicillin resistance), and the phagemid encoding C-FN3contains a different marker (such as kanamycin resistance), so that theyare easily separated.

A binding pair can be added to N-FN3 and C-FN3 in such a way that thebinding pair enhances the association of N-FN3 and C-FN3. This is doneby fusing the gene for one component of the binding partner to the N-FN3gene and the other to the C-FN3 gene using a flexible linker sequence(e.g., poly-Gly) between fused peptides.

Combinatorial libraries of N-FN3 and C-FN3 in appropriate phagemids asdescribed above, in which residues in a loop region are diversified(including insertions and deletions), are made using standard methods(Koide et al., 1998). Phagemid particles for N-FN3 and C-FN3 areseparately generated using a helper phage as described (Koide et al.,1998). Subsequently, E. coli cells (such as XL1-blue, Stratagene)harboring an N-FN3 library are further infected with the phagemidsencoding a C-FN3 library so that in a single E. coli cell both one (ormore) clone of the N-FN3 library and one (or more) clone of the C-FN3library coexists. Phagemid particles are then produced from these cellsunder conditions where N-FN3 and C-FN3 are expressed. N-FN3 and C-FN3associate in the periplasm of E. coli and thus phagemid particlesdisplay the reconstituted FN3 representing one clone from the N-FN3library and one from the C-FN3 library. The phagemid transfectionprocess is very efficient, so that one can construct a large library.

Screening of displayed FN3 is performed using standard methods (Kay etal., 1996; Koide et al., 1998). Note that a phagemid particle containsthe gene for either N-FN3 or C-FN3, and thus it is necessary to recoverat least two phagemid particles to identify the correct combination ofN-FN3 and C-FN3 variants with desired binding function.

Recovered phages are amplified and again used to infect E. coli so thata single E. coli cell harbors both N-FN3 clone and C-FN3 clone. Phagemidparticles are then produced as described above. After a few cycles ofthese selection and amplification processes, genes encoding thecontiguous, full-length monobodies are constructed from the genes forN-FN3 and C-FN3 variants in the selected pool using PCR techniques andcloned into a phagemid vector. Standard phagemid selection experimentsare then performed to identify full-length monobodies with desiredbinding properties.

b. One Vector System.

A phagemid vector expressing N-FN3 fused to a phage coat protein and asecretion signal and C-FN3 fused to a secretion signal (or vise versa)under a single promotor is constructed. A recombinase recognition sitesuch as wild-type lox is introduced in the intergenic region between theN-FN3 and C-FN3 genes. Another recombination site, which is orthogonalto the first one, such as loxP511 is introduced after the two FN3fragment genes. Examples of such phagemid vectors have been described inthe literature (Sblattero and Bradbury 2000; Sblattero et al. 2001).Mutations are introduced in a loop region within the N-FN3 gene usingstandard methods to generate a library of N-FN3. To this ensemble ofphagemid vectors encoding the library, further mutations in a loopregion of the C-FN3 gene are introduced. Phagemid particles are preparedfrom this ensemble of vectors encoding both N-FN3 and C-FN3 librariesusing helper phages. E. coli cells that constitutively expresses anappropriate recombinase, such as Cre recombinase, are infected with thephagemid particles with a high multiplicity of infection so that asingle cell is infected with multiple phagemids. The recombinase in theE. coli cells recombine the phagemids at the recombination sites, thuscreating further diversity. Phagemid particles are then produced fromthese cells and then used to infect another E. coli cell line that doesnot express the recombinase at a low multiplicity of infection. Phagemidparticles are produced from these E. coli cells for library selection.Library selection and amplification of selected phagemids are performedusing standard methods, except that the recombination step can beintroduced to further increase the library diversity.

2. Yeast Two-Hybrid

A binding target (“bait”) is fused to a DNA binding domain, and N-FN3(optionally with a component of a binding pair) is fused to anactivation domain using standard methods (Golemis & Serebriiskii, 1997;Koide et al., 2002). C-FN3 (optionally with the other component of abinding pair) is expressed without fusing it to an activation domainunder a strong promoter such as Gal so that it associates with theN-FN3-activation domain fusion. A library of N-FN3 is constructed inyeast cells of one mating type (e.g., the strain EGY48) and a library ofC-FN3 is constructed in yeast cells of the other mating type (e.g., thestrain RFY206). The bait plasmid is introduced in one of the yeast cellsbefore constructing a library. The two yeast strains are mated and yeasttwo-hybrid screening is performed using standard methods as describedpreviously (Golemis & Serebriiskii, 1997; Koide et al., 2002).Alternatively, C-FN3 can be fused to an activation domain and N-Fn3 canbe expressed without fusing it to an activation domain.

3. Yeast Surface Display.

N-FN3 (optionally with a component of a binding pair) is fused to theAga2 protein in such a way that it allows the surface display of N-FN3(Boder & Wittrup, 1997; Boder & Wittrup, 2000). A vector such as pYD1(Invitrogen) is used for this purpose. C-FN3 (optionally with the othercomponent of a binding pair) is fused to an N-terminal secretionsequence and the gene coding for this fusion protein is placed under anappropriate promoter such as GAL on a vector. Alternatively C-FN3 can bedisplayed on the yeast surface and N-FN3 can be expressed without fusingit to Aga2. A library of N-FN3 is constructed using standard methods(Koide et al., 2002) in the yeast strain EBY100 (Invitrogen™), and alibrary of C-FN3 is constructed in the yeast strain BJ5464 (ATCC). Acollection of EBY100 cells containing a N-FN3 library and a collectionof BJ5464 containing a C-FN3 library are mated to produce diploid cellseach containing one member of the N-FN3 library and one of the C-FN3library. N-FN3 and C-FN3 variants are then displayed on the yeastsurface, and selection of clones are performed as described (Boder &Wittrup, 1997; Boder & Wittrup, 2000).

The following section describes a specific example of yeast surfacedisplay.

The plasmid pYDFN1 was constructed by inserting the FN3 gene into pYD1vector (Invitrogen™), and the gene is expressed under the Gal promoter.The FN3 gene was prepared by PCR, and the termination codon of the FN3gene was removed in such a way that the FN3 gene and the V5 tag sequenceis in frame. N-FN3 (residues 1-42) was fused to the secretion signal ofthe Aga2 protein and the FLAG tag in such a way that it allows thesecretion of N-FN3 followed by the FLAG tag. The secretionsignal-N-FN3-FLAG gene was constructed using PCR, and pYDFN1 was used asa template. The secretion signal-N-FN3-FLAG gene was cloned in theplasmid pRS425 (Sikorski 1989). The resulting plasmid is namedpGalsecFN(N)FLAG. A yeast surface display vector for C-FN3 (residues43-94) was constructed from the plasmid pYDFN1. The DNA segment encodingthe Express tag and residues 1-42 of FN3 was deleted by PCR so that itencodes the Aga2-C-FN3-V5-His fusion protein. The resulting plasmid ispGalAgaFN(C)V5. Schemes of these vectors are shown in FIG. 46. Inaddition, a pGalAgaFN(C)V5 containing a FG-loop from a monobody “STAV11”that binds to streptavidin was constructed (pGalAgaFN(C)V5-STAV11). TheSTAVII clone contains an FG loop sequence of HPMNEKN (SEQ ID NO:138) inplace of the wild-type sequence, RGDSPAS (SEQ ID NO:48).

Yeast EBY100 was transformed with the plasmid pGalsecFN(N)FLAG, andBJ5464 was transformed with the plasmid pGalAgaFN(C)V5 orpGalAgaFN(C)V5-STAV11. These two strains were mated, and the diploidcells were analyzed using fluorescence activated cell sorter (FACS).

Mated cells were grown in YC Glc ura-trp-leu-media, followed by YC GalRaf ura-trp-leu-media in order to induce the expression of the fusionproteins. These media are according to Boder and Wittrup (Boder andWittrup 1997). Cells were spun down, washed with BSS (Tris-Cl pH7.4,NaCl, 1 mg/ml BSA). The cells were mixed with rabbit anti-FLAG antibody(Sigma) and monoclonal anti-V5 antibody (Sigma) in BSS and incubated onice for 40 minutes. The cells were spun down, washed with BSS, and mixedwith anti-rabbit antibody-PE (Sigma) and anti-mouse antibody-FITC(Sigma) in BSS and incubated on ice for 40 minutes. The cells were spundown, washed with BSS, and subjected to a cell sorter (FACScanII™,Beckton Dickinson). In this staining scheme, FITC fluorescence intensityindicates the amount of C-FN3 on the yeast surface and PE intensityindicates the amount of N-FN3 on the surface.

As shown in FIG. 47, the FITC fluorescence monitoring the V5 epitope tagattached to C-FN3 was correlated to the expression of C-FN3 andC-FN3-STAV11. The PE fluorescence monitoring the FLAG tag attached toN-FN3 was correlated to the expression of N-FN3 when C-FN3 wasco-expressed. The surface display of N-FN3 was dependent on C-FN3expression, indicating that N-FN3 and C-FN3 reconstituted on the yeastsurface. These results show that combinatorial libraries can beconstructed from fragment libraries using yeast mating as describedabove.

Once specific pairs of N-FN3 and C-FN3 with desired binding propertiesare identified, genes encoding contiguous, full-length monobodiescontaining the identified loops sequences are constructed from the genesfor the fragments. The genes for such full-length monobodies are clonedinto vectors for library screening and/or into expression vectors, andthese new vectors are used for further library screening and proteinproduction.

The complete disclosure of all patents, patent documents andpublications cited herein are incorporated by reference as ifindividually incorporated. The foregoing detailed description andexamples have been given for clarity of understanding only. Nounnecessary limitations are to be understood therefrom. The invention isnot limited to the exact details shown and described for variationsobvious to one skilled in the art will be included within the inventiondefined by the claims.

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What is claimed is:
 1. A fibronectin monobody polypeptide comprising afirst fibronectin type III (Fn3) monobody fragment having at least twoFn3 beta-strand domains with a loop region linked between eachbeta-strand domain, and at least one additional fibronectin monobodyfragment associated with the first Fn3 monobody fragment, comprising atleast two fibronectin beta strand domains with a loop region linkedbetween each beta-strand domain, wherein the loop regions of the firstFn3 monobody fragment and the at least one additional fibronectinmonobody fragment correspond to wild-type fibronectin loop regions, orcomprise an AB, BC, CD, DE, EF or FG loop region sequence that israndomized or varies from a corresponding wild-type fibronectin loopregion by deletion of one to all except one amino acid in the loopregion, insertion of one to 25 amino acids, and/or replacement of atleast one amino acid in the loop region, wherein at least one loopregion from the first fibronectin type III (Fn3) monobody fragment orthe at least one additional fibronectin monobody fragment is capable ofbinding to a specific binding partner (SBP) to form a polypeptide:SBPcomplex.
 2. The monobody of claim 1, wherein the first fibronectin typeIII (Fn3) monobody fragment comprises at least two beta-strand domainsfrom a tenth fibronectin type III module (Fn3¹⁰) and the additionalfibronectin monobody fragment comprises at least two beta-strand domainsfrom a fibronectin type III (Fn3) polypeptide.
 3. The monobody of claim1, wherein the first fibronectin type III (Fn3) monobody fragmentcomprises at least two beta-strand domains from a tenth fibronectin typeIII module)(Fn3¹⁰ and the at least one additional fibronectin monobodyfragment comprises at least two beta-strand domains from a fibronectintype II (Fn2) polypeptide.
 4. The monobody of claim 1, wherein at leastone loop region of the first fibronectin type III (Fn3) monobodyfragment comprises amino acid residues: i) from 15 to 16 inclusive in anAB loop; ii) from 22 to 30 inclusive in a BC loop; iii) from 39 to 45inclusive in a CD loop; iv) from 51 to 55 inclusive in a DE loop; v)from 60 to 66 inclusive in an EF loop; or vi) from 76 to 87 inclusive inan FG loop.
 5. The monobody of claim 1, wherein at least one loop regionof the first fibronectin type III (Fn3) monobody fragment varies fromthe corresponding wild-type Fn3 loop region by deletion of one to allexcept one amino acid in the loop region, insertion of one to 25 aminoacids, and/or replacement of at least one amino acid in the loop region.6. The monobody of claim 1, wherein at least one loop region of thefirst fibronectin type III (Fn3) monobody fragment varies from thecorresponding wild-type Fn3 loop region by deletion of one to all exceptone amino acid and/or replacement of at least one amino acid.
 7. Themonobody of claim 1, wherein at least one loop region of the firstfibronectin type III (Fn3) monobody fragment varies from thecorresponding wild-type Fn3 loop region by insertion of one to 25 aminoacids.
 8. The monobody of claim 1, wherein at least one loop region fromthe first fibronectin type III (Fn3) monobody fragment or the at leastone additional fibronectin monobody fragment comprises an antibody CDRsequence, wherein the monobody binds to a specific binding partner. 9.The monobody of claim 1, wherein the monobody comprises a first Fn3monobody fragment having at least two to five Fn3 beta-strand domains inthe Fn3 wild-type arrangement with a loop region linked between eachbeta-strand domain, and at least one additional complementaryfibronectin monobody fragment comprising at least two to fivefibronectin beta strand domains in the wild-type arrangement with a loopregion linked between each beta-strand domain.